:root {
  /* ── Colors ── */
  --color-text-primary: #1a1a1a;
  --color-text-secondary: #444;
  --color-text-tertiary: #777;
  --color-background-primary: #fff;
  --color-background-secondary: #f5f5f5;
  --color-border-primary: #333;
  --color-border-secondary: #d0d0d0;
  --color-border-tertiary: #e8e8e8;

  /* ── Badge colors ── */
  --badge-ds-bg: #eeedfe;
  --badge-ds-fg: #3c3489;
  --badge-graph-bg: #faece7;
  --badge-graph-fg: #712b13;
  --badge-dp-bg: #faeeda;
  --badge-dp-fg: #633806;
  --badge-tree-bg: #eaf3de;
  --badge-tree-fg: #27500a;
  --badge-sort-bg: #e6f1fb;
  --badge-sort-fg: #0c447c;

  /* ── Typography ── */
  --font-mono: 'SF Mono', 'Consolas', monospace;
  --font-size-xs: 11px;
  --font-size-sm: 12px;
  --font-size-md: 13px;
  --font-size-base: 14px;
  --font-size-lg: 15px;
  --font-weight-normal: 400;
  --font-weight-medium: 500;

  /* ── Spacing ── */
  --space-xs: 4px;
  --space-sm: 6px;
  --space-md: 8px;
  --space-lg: 10px;
  --space-xl: 12px;
  --space-2xl: 14px;
  --space-3xl: 16px;
  --space-4xl: 1rem;
  --space-5xl: 1.5rem;
  --space-6xl: 1.8rem;
  --space-7xl: 2rem;

  /* ── Border radius ── */
  --radius-sm: 6px;
  --radius-md: 8px;
  --radius-lg: 12px;
  --radius-pill: 20px;

  /* ── Transitions ── */
  --transition-fast: 0.12s;
  --transition-normal: 0.15s;
  --transition-slow: 0.2s;

  /* ── Layout ── */
  --grid-min-col: 290px;
  --grid-gap: var(--space-xl);
  --card-padding: var(--space-4xl) 1.1rem;
  --line-height: 1.6;

  /* ── System Preferences ── */
  color-scheme: light dark;
}

/* ── Dark mode (data-theme) ── */
:root[data-theme="dark"] {
  /* ── Base colors ── */
  --color-text-primary: #e8e8e8;
  --color-text-secondary: #b8b8b8;
  --color-text-tertiary: #8a8a8a;
  --color-background-primary: #1a1a1a;
  --color-background-secondary: #232323;
  --color-border-primary: #555;
  --color-border-secondary: #3c3c3c;
  --color-border-tertiary: #2d2d2d;

  /* ── Badges ── */
  --badge-ds-bg: #2a2854;
  --badge-ds-fg: #b8b3f0;
  --badge-graph-bg: #40261d;
  --badge-graph-fg: #f0a88a;
  --badge-dp-bg: #403114;
  --badge-dp-fg: #edc478;
  --badge-tree-bg: #213810;
  --badge-tree-fg: #a8d47a;
  --badge-sort-bg: #163252;
  --badge-sort-fg: #7ab8e8;
}

/* ── Reset (scoped) ── */
html,
body {
  background-color: var(--color-background-primary);
  color: var(--color-text-primary);
}

/* ── Hero Page ── */

.hero-page {
  min-height: 100vh;
  background: var(--color-background-primary);
}

.hero-shell {
  max-width: 560px;
  margin: 0 auto;
  padding: 24px 28px;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

.hero-topbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.hero-logo {
  font-size: 18px;
  font-weight: 600;
  color: var(--color-text-tertiary);
  letter-spacing: -0.01em;
  font-family: var(--font-mono);
  text-decoration: none;
  transition: color 0.15s;
  cursor: pointer;
}

.hero-logo:hover {
  color: var(--color-text-primary);
}

.hero-center {
  flex: 1;
  display: flex;
  flex-direction: column;
  justify-content: center;
  padding: 80px 0 60px;
}

.hero-eyebrow {
  font-size: 11px;
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--color-text-tertiary);
  margin-bottom: 16px;
}

.hero-heading {
  font-size: clamp(2.2rem, 6vw, 3.2rem);
  font-weight: 700;
  letter-spacing: -0.04em;
  line-height: 1.05;
  color: var(--color-text-primary);
  margin-bottom: 14px;
}

.hero-desc {
  font-size: 14.5px;
  line-height: 1.65;
  color: var(--color-text-secondary);
  margin-bottom: 44px;
  max-width: 420px;
}

/* ── Hero Navigation links ── */
.hero-nav {
  display: flex;
  flex-direction: column;
  gap: 1px;
  background: var(--color-border-tertiary);
  border: 1px solid var(--color-border-secondary);
  border-radius: 12px;
  overflow: hidden;
}

.hero-link {
  display: grid;
  grid-template-columns: 1fr auto;
  grid-template-rows: auto auto;
  gap: 4px 20px;
  padding: 20px 24px;
  background: var(--color-background-primary);
  text-decoration: none;
  color: inherit;
  transition: background 0.15s;
}

.hero-link:hover {
  background: var(--color-background-secondary);
}

.hero-link-title {
  font-size: 15px;
  font-weight: 600;
  color: var(--color-text-primary);
  grid-column: 1;
  grid-row: 1;
}

.hero-link-meta {
  font-size: 12.5px;
  color: var(--color-text-tertiary);
  grid-column: 1;
  grid-row: 2;
}

.hero-link-arrow {
  grid-column: 2;
  grid-row: 1 / -1;
  align-self: center;
  font-size: 18px;
  color: var(--color-text-tertiary);
  transition: transform 0.2s, color 0.2s;
}

.hero-link:hover .hero-link-arrow {
  transform: translateX(4px);
  color: var(--color-text-primary);
}

/* ── Hero Footer ── */
.hero-foot {
  padding: 24px 0;
  text-align: center;
  font-size: 12px;
  color: var(--color-text-tertiary);
}

.hero-foot a {
  color: var(--color-text-tertiary);
  text-decoration: none;
  transition: color 0.15s;
}

.hero-foot a:hover {
  color: var(--color-text-secondary);
}

/* ── Hero responsive ── */
@media (max-width: 640px) {
  .hero-shell {
    padding: 20px 20px;
  }

  .hero-center {
    padding: 60px 0 40px;
  }

  .hero-link {
    padding: 16px 18px;
  }
}

/* ── Utilities ── */
.btn-interactive {
  cursor: pointer;
  touch-action: manipulation;
  transition: background var(--transition-normal), color var(--transition-normal), border-color var(--transition-normal);
}

.btn-interactive:hover {
  background: var(--color-background-secondary);
}

.btn-interactive:active {
  opacity: 0.7;
}

.btn-interactive:focus-visible {
  outline: 2px solid var(--color-border-primary);
  outline-offset: 2px;
}

.wrap,
.wrap * {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  -webkit-tap-highlight-color: transparent;
}

.ds-wrap {
  padding: 76px var(--space-4xl) var(--space-4xl);
  scroll-behavior: smooth;
}

/* ── App Header ── */
.ds-header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  z-index: 50;
  background: var(--color-background-primary);
  border-bottom: 0.5px solid var(--color-border-tertiary);
  display: flex;
  justify-content: space-between;
  align-items: center;
  height: 60px;
  padding: 0 var(--space-4xl);
}

.ds-app-title {
  font-size: 1.5rem;
  font-weight: 600;
  color: var(--color-text-primary);
}


/* ── Header Actions ── */
.ds-header-actions,
.algo-header-actions {
  display: flex;
  align-items: center;
  gap: var(--space-md);
}

.ds-nav-link,
.algo-nav-link {
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-secondary);
  text-decoration: none;
  padding: var(--space-sm) var(--space-xl);
  border: 0.5px solid var(--color-border-secondary);
  border-radius: var(--radius-pill);
}

.ds-nav-link:hover,
.algo-nav-link:hover {
  color: var(--color-text-primary);
  border-color: var(--color-border-primary);
}

/* ── Theme Toggle ── */
.hero-theme-toggle,
.ds-theme-toggle,
.algo-theme-toggle {
  background: transparent;
  border: none;
  font-size: 1.25rem;
  padding: var(--space-sm);
  border-radius: var(--radius-md);
  color: var(--color-text-primary);
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 40px;
  min-width: 40px;
  -webkit-tap-highlight-color: transparent;
}

.hero-icon-sun,
.ds-icon-sun,
.algo-icon-sun {
  display: none;
}

.hero-icon-moon,
.ds-icon-moon,
.algo-icon-moon {
  display: block;
}

:root[data-theme="dark"] .hero-icon-sun,
:root[data-theme="dark"] .ds-icon-sun,
:root[data-theme="dark"] .algo-icon-sun {
  display: block;
}

:root[data-theme="dark"] .hero-icon-moon,
:root[data-theme="dark"] .ds-icon-moon,
:root[data-theme="dark"] .algo-icon-moon {
  display: none;
}

/* ── Tabs ── */
.ds-tabs {
  display: flex;
  gap: var(--space-md);
  flex-wrap: wrap;
  margin-bottom: var(--space-5xl);
}

.ds-tab {
  padding: var(--space-md) var(--space-3xl);
  min-height: 40px;
  border-radius: var(--radius-pill);
  border: 0.5px solid var(--color-border-secondary);
  background: transparent;
  font-size: var(--font-size-md);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-secondary);
}

.ds-tab.ds-active {
  background: var(--color-text-primary);
  color: var(--color-background-primary);
  border-color: var(--color-text-primary);
}

/* ── Search ── */
.ds-wrap search {
  display: block;
  margin-bottom: var(--space-6xl);
}

.ds-search-input {
  width: 100%;
  padding: var(--space-lg) var(--space-2xl);
  min-height: 40px;
  border: 0.5px solid var(--color-border-secondary);
  border-radius: var(--radius-md);
  font-size: 16px;
  /* prevents iOS zoom on focus */
  background: var(--color-background-primary);
  color: var(--color-text-primary);
  transition: border-color var(--transition-normal);
}

.ds-search-input:focus {
  outline: none;
  border-color: var(--color-border-primary);
}

/* ── Grid ── */
.ds-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(var(--grid-min-col), 1fr));
  gap: var(--grid-gap);
  align-items: start;
}

/* ── Card ── */
.ds-card {
  background: var(--color-background-primary);
  border: 0.5px solid var(--color-border-tertiary);
  border-radius: var(--radius-lg);
  padding: 0;
  cursor: pointer;
  touch-action: manipulation;
  transition: border-color var(--transition-normal);
  overflow: hidden;
}

.ds-card:active {
  opacity: 0.85;
}

.ds-card:hover {
  border-color: var(--color-border-primary);
}

.ds-card:focus-visible {
  outline: 2px solid var(--color-border-primary);
  outline-offset: 2px;
}

.ds-card.ds-open {
  border-color: var(--color-border-primary);
  grid-column: 1 / -1;
}

.ds-card-head {
  display: flex;
  justify-content: center;
  align-items: center;
  gap: var(--space-lg);
  padding: var(--space-xl) 1.1rem;
  position: relative;
}

/* ── Badges ── */
.ds-badge {
  font-size: var(--font-size-xs);
  font-weight: var(--font-weight-medium);
  padding: 3px 9px;
  border-radius: var(--radius-pill);
  white-space: nowrap;
}

.badge-ds {
  background: var(--badge-ds-bg);
  color: var(--badge-ds-fg);
}

.badge-graph {
  background: var(--badge-graph-bg);
  color: var(--badge-graph-fg);
}

.badge-dp {
  background: var(--badge-dp-bg);
  color: var(--badge-dp-fg);
}

.badge-tree {
  background: var(--badge-tree-bg);
  color: var(--badge-tree-fg);
}

.badge-sort {
  background: var(--badge-sort-bg);
  color: var(--badge-sort-fg);
}

.ds-card-title {
  font-size: var(--font-size-base);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-primary);
  flex: 1;
  margin: 0;
}

.ds-chevron,
.algo-chevron {
  font-size: var(--font-size-sm);
  color: var(--color-text-tertiary);
  transition: transform var(--transition-slow);
}

.ds-card.ds-open .ds-chevron {
  transform: rotate(90deg);
}

.ds-card-body {
  display: grid;
  grid-template-rows: 0fr;
  opacity: 0;
  visibility: hidden;
  margin-top: 0;
  border-top: 0.5px solid transparent;
  transition: grid-template-rows 0.3s cubic-bezier(0.4, 0, 0.2, 1),
    margin-top 0.3s cubic-bezier(0.4, 0, 0.2, 1),
    border-color 0.3s cubic-bezier(0.4, 0, 0.2, 1),
    opacity 0.2s ease-out,
    visibility 0.3s;
}

.ds-card.ds-open .ds-card-body {
  grid-template-rows: 1fr;
  opacity: 1;
  visibility: visible;
  margin-top: var(--space-xl);
  border-top-color: var(--color-border-tertiary);
}

.ds-card-inner {
  overflow: hidden;
  min-height: 0;
  padding-top: var(--space-xl);
}

.ds-card-inner {
  padding: 0 1.1rem 1.1rem;
}

.ds-section-label,
.algo-section-label {
  font-size: var(--font-size-xs);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-tertiary);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  margin: var(--space-lg) 0 var(--space-xs);
}

.ds-desc,
.algo-desc {
  font-size: var(--font-size-md);
  color: var(--color-text-secondary);
  line-height: var(--line-height);
}

/* ── Operations (<dl>) ── */
.ds-ops {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-xs);
  margin-top: var(--space-xs);
}

.ds-op-item {
  display: flex;
  justify-content: space-between;
  padding: var(--space-xs) var(--space-md);
  font-size: var(--font-size-sm);
  background: var(--color-background-secondary);
  border-radius: var(--radius-sm);
}

.ds-op-name {
  color: var(--color-text-secondary);
}

.algo-tc-best,
.algo-tc-avg,
.algo-tc-worst {
  font-weight: var(--font-weight-medium);
}

.algo-tc-space {
  vertical-align: middle;
}

.ds-op-tc {
  font-weight: var(--font-weight-medium);
  font-size: var(--font-size-xs);
  font-family: var(--font-mono);
  color: var(--color-text-primary);
}

/* ── Use-case tags (<ul>/<li>) ── */
.ds-uses-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-xs);
  margin-top: var(--space-xs);
  list-style: none;
}

.ds-use-tag {
  font-size: var(--font-size-xs);
  padding: var(--space-sm) var(--space-md);
  min-height: 32px;
  display: flex;
  align-items: center;
  background: var(--color-background-secondary);
  border-radius: var(--radius-sm);
  color: var(--color-text-secondary);
  line-height: 1.4;
  cursor: pointer;
  touch-action: manipulation;
  border: 0.5px solid transparent;
  transition: border-color var(--transition-fast),
    color var(--transition-fast);
}

.ds-use-tag:hover {
  border-color: var(--color-border-secondary);
  color: var(--color-text-primary);
}

.ds-use-tag:active {
  opacity: 0.7;
}

.ds-use-tag:focus-visible {
  outline: 2px solid var(--color-border-primary);
  outline-offset: 1px;
}

/* ── Ask button ── */
.ds-practice-btn {
  margin-top: var(--space-xl);
  padding: var(--space-md) var(--space-2xl);
  min-height: 40px;
  border: 0.5px solid var(--color-border-secondary);
  border-radius: var(--radius-md);
  background: transparent;
  font-size: var(--font-size-sm);
  color: var(--color-text-secondary);
}

/* ── Empty state ── */
.ds-empty-msg {
  text-align: center;
  padding: var(--space-7xl);
  color: var(--color-text-tertiary);
  font-size: var(--font-size-base);
  grid-column: 1 / -1;
}

/* ── Footer ── */
.ds-footer,
.algo-footer {
  text-align: center;
  margin-top: var(--space-5xl);
  padding-top: var(--space-3xl);
  font-size: var(--font-size-sm);
  color: var(--color-text-tertiary);
}

.ds-footer a,
.algo-footer a {
  color: var(--color-text-secondary);
  text-decoration: none;
  font-weight: var(--font-weight-medium);
  transition: color var(--transition-fast);
}

.ds-footer a:hover,
.algo-footer a:hover {
  text-decoration: underline;
  color: var(--color-text-primary);
}

/* ── Mobile responsive ── */
@media (max-width: 640px) {
  :root {
    --grid-min-col: 100%;
  }

  .ds-wrap {
    padding: 76px var(--space-3xl) var(--space-4xl);
  }

  .ds-tab {
    padding: var(--space-md) var(--space-2xl);
    font-size: var(--font-size-base);
  }

  .ds-ops {
    grid-template-columns: 1fr;
  }

  .ds-uses-grid {
    grid-template-columns: 1fr;
  }
}

/* ── Docs page layout ── */
.algo-page {
  display: flex;
  min-height: 100vh;
}

/* ── Sidebar ── */
.algo-sidebar {
  --sidebar-w: 260px;
  width: var(--sidebar-w);
  min-height: 100vh;
  background: var(--color-background-primary);
  border-right: 0.5px solid var(--color-border-secondary);
  position: fixed;
  top: 0;
  left: 0;
  overflow-y: auto;
  z-index: 100;
  display: flex;
  flex-direction: column;
}

.algo-sidebar-header {
  height: 60px;
  display: flex;
  align-items: center;
  padding: 0 20px;
  border-bottom: 0.5px solid var(--color-border-tertiary);
  position: sticky;
  top: 0;
  background: var(--color-background-primary);
  z-index: 10;
}

.algo-sidebar-title {
  font-size: 1.1rem;
  font-weight: 600;
  color: var(--color-text-primary);
  margin: var(--space-md) 0 2px;
}

.algo-sidebar-subtitle {
  font-size: var(--font-size-xs);
  color: var(--color-text-tertiary);
  font-family: var(--font-mono);
}

.back-link {
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-tertiary);
  text-decoration: none;
  transition: color var(--transition-fast);
}

.back-link:hover {
  color: var(--color-text-primary);
}

.algo-search {
  width: calc(100% - 32px);
  margin: var(--space-xl) 16px var(--space-md);
  padding: var(--space-md) var(--space-xl);
  background: var(--color-background-secondary);
  border: 0.5px solid var(--color-border-secondary);
  border-radius: var(--radius-sm);
  font-size: var(--font-size-sm);
  color: var(--color-text-primary);
  outline: none;
  transition: border-color var(--transition-normal);
}

.algo-search:focus {
  border-color: var(--color-border-primary);
}

/* ── Nav tree ── */
.algo-nav-group {
  padding: var(--space-md) 0;
}

.algo-nav-label {
  font-size: 10px;
  font-weight: var(--font-weight-medium);
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--color-text-tertiary);
  padding: var(--space-sm) 20px;
}

.algo-nav-item {
  display: flex;
  align-items: center;
  gap: var(--space-md);
  padding: var(--space-sm) 20px;
  font-size: var(--font-size-sm);
  cursor: pointer;
  color: var(--color-text-secondary);
  text-decoration: none;
  border-left: 2px solid transparent;
  transition: all var(--transition-fast);
}

.algo-nav-item:hover {
  color: var(--color-text-primary);
  background: var(--color-background-secondary);
}

.algo-nav-item.algo-active {
  color: var(--color-text-primary);
  border-left-color: var(--color-text-primary);
  font-weight: var(--font-weight-medium);
}

.algo-nav-dot {
  width: 6px;
  height: 6px;
  border-radius: 50%;
  flex-shrink: 0;
  background: var(--topic-accent-light, var(--color-text-tertiary));
}

:root[data-theme="dark"] .algo-nav-dot {
  background: var(--topic-accent-dark, var(--color-text-tertiary));
}

/* ── Main content ── */
.algo-main {
  margin-left: 260px;
  flex: 1;
  min-height: 100vh;
}

.algo-topbar {
  position: fixed;
  top: 0;
  left: 260px;
  right: 0;
  background: var(--color-background-primary);
  border-bottom: 0.5px solid var(--color-border-tertiary);
  height: 60px;
  padding: 0 var(--space-lg);
  z-index: 50;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.algo-topbar-left {
  display: flex;
  align-items: center;
  gap: var(--space-xl);
}

.algo-menu-btn {
  display: none;
  background: none;
  border: 0.5px solid var(--color-border-secondary);
  color: var(--color-text-primary);
  padding: var(--space-sm) var(--space-lg);
  border-radius: var(--radius-sm);
  cursor: pointer;
  font-size: 16px;
}

.algo-breadcrumb {
  font-size: var(--font-size-md);
  color: var(--color-text-tertiary);
  font-family: var(--font-mono);
}

.algo-breadcrumb strong {
  color: var(--color-text-primary);
}

.algo-stats {
  display: flex;
  align-items: center;
  gap: var(--space-3xl);
  font-size: var(--font-size-sm);
  color: var(--color-text-tertiary);
}

.algo-stats strong {
  color: var(--color-text-primary);
}

.algo-content {
  padding: 80px 40px 80px;
  max-width: 1000px;
}

/* ── Hero ── */
.algo-hero {
  margin-bottom: 48px;
  padding-bottom: 36px;
  border-bottom: 0.5px solid var(--color-border-tertiary);
}

.algo-hero-tag {
  font-size: var(--font-size-xs);
  font-family: var(--font-mono);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--color-text-tertiary);
  margin-bottom: var(--space-xl);
}

.algo-hero-title {
  font-size: clamp(28px, 5vw, 48px);
  font-weight: 600;
  line-height: 1.15;
  color: var(--color-text-primary);
  margin-bottom: var(--space-xl);
}

.algo-hero-desc {
  font-size: var(--font-size-base);
  color: var(--color-text-secondary);
  max-width: 540px;
  line-height: 1.65;
}

.algo-hero-stats {
  display: flex;
  gap: var(--space-3xl);
  margin-top: 24px;
  flex-wrap: wrap;
}

.algo-stat {
  background: var(--color-background-secondary);
  border: 0.5px solid var(--color-border-tertiary);
  border-radius: var(--radius-md);
  padding: var(--space-lg) var(--space-3xl);
  text-align: center;
}

.algo-stat-num {
  display: block;
  font-size: 1.3rem;
  font-weight: 600;
  color: var(--color-text-primary);
  font-family: var(--font-mono);
}

.algo-stat-label {
  font-size: var(--font-size-xs);
  color: var(--color-text-tertiary);
  text-transform: uppercase;
  letter-spacing: 0.06em;
}

/* ── Topic section ── */
.algo-topic {
  margin-bottom: 30px;
}

.algo-topic-head {
  display: flex;
  align-items: center;
  gap: var(--space-xl);
  margin-bottom: 24px;
  padding-bottom: var(--space-xl);
  border-bottom: 0.5px solid var(--color-border-tertiary);
}

.algo-topic-icon {
  width: 32px;
  height: 32px;
  border-radius: var(--radius-sm);
  flex-shrink: 0;
  background: var(--topic-accent-light, var(--color-text-tertiary));
  opacity: 0.15;
}

:root[data-theme="dark"] .algo-topic-icon {
  background: var(--topic-accent-dark, var(--color-text-tertiary));
}

.algo-topic-head h2 {
  font-size: 1.2rem;
  font-weight: 600;
  color: var(--color-text-primary);
  flex: 1;
}

.algo-topic-count {
  font-size: var(--font-size-xs);
  font-family: var(--font-mono);
  color: var(--color-text-tertiary);
  background: var(--color-background-secondary);
  border: 0.5px solid var(--color-border-tertiary);
  border-radius: var(--radius-pill);
  padding: 2px var(--space-lg);
}

/* ── Algo card ── */
.algo-card {
  background: var(--color-background-primary);
  border: 0.5px solid var(--color-border-tertiary);
  border-radius: var(--radius-lg);
  margin-bottom: var(--space-xl);
  overflow: hidden;
  transition: border-color var(--transition-normal);
}

.algo-card:hover {
  border-color: var(--color-border-secondary);
}

.algo-card-head {
  display: flex;
  align-items: center;
  gap: var(--space-xl);
  padding: var(--space-lg) 1.1rem;
  cursor: pointer;
  user-select: none;
}

.algo-card-name {
  font-size: var(--font-size-lg);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-primary);
  flex: 1;
  margin: 0;
}

.algo-card-badges {
  display: flex;
  gap: var(--space-sm);
  flex-wrap: wrap;
}

.algo-card-badge {
  font-size: var(--font-size-xs);
  font-weight: var(--font-weight-medium);
  padding: 3px 9px;
  border-radius: var(--radius-pill);
  white-space: nowrap;
}

.algo-chevron {
  font-size: var(--font-size-sm);
  color: var(--color-text-tertiary);
  transition: transform var(--transition-slow);
}

.algo-card.algo-open .algo-chevron {
  transform: rotate(90deg);
}

.algo-card-body {
  display: grid;
  grid-template-rows: 0fr;
  opacity: 0;
  visibility: hidden;
  overflow: hidden;
  border-top: 0.5px solid transparent;
  transition:
    grid-template-rows 0.3s cubic-bezier(0.4, 0, 0.2, 1),
    border-color 0.3s cubic-bezier(0.4, 0, 0.2, 1),
    opacity 0.2s ease-out,
    visibility 0.3s;
}

.algo-card.algo-open .algo-card-body {
  grid-template-rows: 1fr;
  opacity: 1;
  visibility: visible;
  border-top-color: var(--color-border-tertiary);
}

.algo-card-inner {
  overflow: hidden;
  min-height: 0;
  padding: 0 1.1rem;
  transition: padding 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

.algo-card.algo-open .algo-card-inner {
  padding: var(--space-xl) 1.1rem 1.1rem;
}

/* ── Approach steps ── */
.algo-steps {
  counter-reset: step;
  list-style: none;
  padding: 0;
  display: flex;
  flex-direction: column;
  gap: var(--space-sm);
  margin-top: var(--space-xs);
}

.algo-steps li {
  counter-increment: step;
  font-size: var(--font-size-md);
  color: var(--color-text-secondary);
  display: flex;
  gap: var(--space-lg);
  align-items: flex-start;
  line-height: 1.6;
}

.algo-steps li::before {
  content: counter(step);
  background: var(--color-background-secondary);
  border: 0.5px solid var(--color-border-tertiary);
  color: var(--color-text-primary);
  font-family: var(--font-mono);
  font-size: 10px;
  font-weight: var(--font-weight-medium);
  width: 20px;
  height: 20px;
  border-radius: 4px;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
  margin-top: 2px;
}

/* ── Complexity table ── */
.algo-complexity {
  width: 100%;
  border-collapse: collapse;
  font-family: var(--font-mono);
  font-size: var(--font-size-sm);
  margin-top: var(--space-xs);
}

.algo-complexity th {
  text-align: left;
  color: var(--color-text-tertiary);
  font-size: 10px;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  padding: var(--space-sm) var(--space-lg);
  background: var(--color-background-secondary);
  border: 0.5px solid var(--color-border-tertiary);
}

.algo-complexity td {
  padding: var(--space-sm) var(--space-lg);
  border: 0.5px solid var(--color-border-tertiary);
  color: var(--color-text-secondary);
}

.algo-complexity tr:hover td {
  background: var(--color-background-secondary);
}

/* ── Code block ── */
.algo-code-wrapper {
  border: 0.5px solid var(--color-border-secondary);
  border-radius: var(--radius-md);
  overflow: hidden;
  margin-top: var(--space-xs);
  background: var(--color-background-secondary);
}

.algo-code-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-sm) var(--space-xl);
  background: var(--color-background-secondary);
  border-bottom: 0.5px solid var(--color-border-tertiary);
}

.algo-code-lang {
  font-family: var(--font-mono);
  font-size: var(--font-size-xs);
  font-weight: var(--font-weight-medium);
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--color-text-tertiary);
}

.algo-code-copy {
  font-family: var(--font-mono);
  font-size: var(--font-size-xs);
  color: var(--color-text-tertiary);
  background: var(--color-background-primary);
  border: 0.5px solid var(--color-border-secondary);
  border-radius: 4px;
  padding: 3px var(--space-md);
  cursor: pointer;
  transition: color var(--transition-fast), border-color var(--transition-fast);
}

.algo-code-copy:hover,
.algo-code-copy.algo-copied {
  color: var(--color-text-primary);
  border-color: var(--color-border-primary);
}

.algo-code-scroll {
  overflow-x: auto;
  overflow-y: auto;
  max-height: 320px;
  -webkit-overflow-scrolling: touch;
}

.algo-code-block {
  font-family: var(--font-mono);
  font-size: var(--font-size-sm);
  line-height: 1.75;
  color: var(--color-text-primary);
  display: table;
  min-width: 100%;
  padding: var(--space-lg) 0;
  margin: 0;
  white-space: pre;
}

.algo-code-line {
  display: table-row;
}

.algo-code-line:hover .algo-code-ln,
.algo-code-line:hover .algo-code-text {
  background: var(--color-background-primary);
}

.algo-code-ln {
  display: table-cell;
  user-select: none;
  text-align: right;
  padding: 0 var(--space-lg) 0 var(--space-xl);
  color: var(--color-text-tertiary);
  font-size: var(--font-size-xs);
  min-width: 32px;
  border-right: 0.5px solid var(--color-border-tertiary);
  vertical-align: top;
  opacity: 0.5;
}

.algo-code-text {
  display: table-cell;
  padding: 0 var(--space-xl) 0 var(--space-xl);
  white-space: pre;
  vertical-align: top;
}

/* ── Syntax highlight colors ── */
.algo-code-kw {
  color: #0550ae;
  font-weight: var(--font-weight-medium);
}

.algo-code-tp {
  color: #6f42c1;
}

.algo-code-fn {
  color: #1a7f37;
}

.algo-code-str {
  color: #0a3069;
}

.algo-code-num {
  color: #0550ae;
}

.algo-code-cm {
  color: var(--color-text-tertiary);
  font-style: italic;
}

:root[data-theme="dark"] .algo-code-kw {
  color: #79c0ff;
}

:root[data-theme="dark"] .algo-code-tp {
  color: #d2a8ff;
}

:root[data-theme="dark"] .algo-code-fn {
  color: #7ee787;
}

:root[data-theme="dark"] .algo-code-str {
  color: #a5d6ff;
}

:root[data-theme="dark"] .algo-code-num {
  color: #79c0ff;
}

:root[data-theme="dark"] .algo-code-cm {
  color: #8b949e;
}

/* ── Tip box ── */
.algo-tip {
  background: var(--color-background-secondary);
  border: 0.5px solid var(--color-border-tertiary);
  border-radius: var(--radius-md);
  padding: var(--space-xl) var(--space-3xl);
  font-size: var(--font-size-md);
  color: var(--color-text-secondary);
  line-height: 1.6;
}

/* ── Practice questions ── */
.algo-questions {
  list-style: none;
  padding: 0;
  display: flex;
  flex-direction: column;
  gap: var(--space-sm);
  margin-top: var(--space-xs);
}

.algo-question {
  display: flex;
  align-items: center;
  gap: var(--space-lg);
  font-size: var(--font-size-sm);
  color: var(--color-text-secondary);
  padding: var(--space-sm) var(--space-lg);
  background: var(--color-background-secondary);
  border-radius: var(--radius-sm);
  border: 0.5px solid var(--color-border-tertiary);
  cursor: pointer;
  transition: background-color var(--transition-fast), border-color var(--transition-fast);
}

.algo-question:hover {
  background: var(--color-background-primary);
  border-color: var(--color-border-secondary);
}

.algo-diff {
  font-family: var(--font-mono);
  font-size: 9px;
  padding: 2px 6px;
  border-radius: 3px;
  font-weight: var(--font-weight-medium);
  flex-shrink: 0;
  letter-spacing: 0.04em;
  text-transform: uppercase;
}

.diff-easy {
  background: var(--badge-tree-bg);
  color: var(--badge-tree-fg);
}

.diff-med {
  background: var(--badge-dp-bg);
  color: var(--badge-dp-fg);
}

.diff-hard {
  background: var(--badge-graph-bg);
  color: var(--badge-graph-fg);
}

.algo-tag {
  font-family: var(--font-mono);
  font-size: 9px;
  color: var(--color-text-tertiary);
  margin-left: auto;
  white-space: nowrap;
}

/* ── Scroll to top ── */
.algo-scrolltop {
  position: fixed;
  bottom: 28px;
  right: 28px;
  width: 36px;
  height: 36px;
  background: var(--color-text-primary);
  color: var(--color-background-primary);
  border: none;
  border-radius: var(--radius-md);
  font-size: 16px;
  cursor: pointer;
  display: none;
  align-items: center;
  justify-content: center;
  z-index: 200;
  transition: opacity var(--transition-normal);
}

.algo-scrolltop.algo-show {
  display: flex;
}

/* ── Docs page scrollbar ── */
.algo-page ::-webkit-scrollbar {
  width: 5px;
  height: 5px;
}

.algo-page ::-webkit-scrollbar-track {
  background: var(--color-background-primary);
}

.algo-page ::-webkit-scrollbar-thumb {
  background: var(--color-border-secondary);
  border-radius: 3px;
}

.algo-page ::-webkit-scrollbar-thumb:hover {
  background: var(--color-border-primary);
}

/* ── Docs mobile ── */
@media (max-width: 768px) {
  .algo-sidebar {
    transform: translateX(-100%);
    transition: transform 0.3s;
  }

  .algo-sidebar.algo-open {
    transform: translateX(0);
  }

  .algo-main {
    margin-left: 0;
  }

  .algo-content {
    padding: 80px 20px 60px;
  }

  .algo-topbar {
    left: 0;
    padding: var(--space-xl) 20px;
  }

  .ds-header {
    padding: 0 20px;
  }

  .algo-menu-btn {
    display: flex !important;
  }

  .algo-hero-stats {
    gap: var(--space-md);
  }

  .algo-stat {
    padding: var(--space-md) var(--space-xl);
  }

  /* ── Responsive code blocks ── */
  .algo-code-block {
    font-size: 11px;
  }

  .algo-code-ln {
    padding: 0 6px 0 8px;
    min-width: 24px;
    font-size: 9px;
  }

  .algo-code-text {
    padding: 0 10px 0 8px;
  }

  .algo-stats {
    display: none;
  }
}

/* ── Very small screens (hide line numbers) ── */
@media (max-width: 480px) {
  .algo-code-ln {
    display: none;
  }

  .algo-code-text {
    padding: 0 12px;
  }

  .algo-code-block {
    white-space: pre-wrap;
    word-break: break-all;
  }
}