:root{--bg:#fdfcf9;--fg:#1e1e1e;--muted:#5f5f5f;--line:#e4e1d8;--link:#0b57d0;--card:#ffffff;--code-bg:#f4f2ec;--easy:#1a7f37;--medium:#b35900;--hard:#c62828;--accent:#8a5a00}
@media (prefers-color-scheme:dark){:root{--bg:#121417;--fg:#e8e6e3;--muted:#a3a3a3;--line:#2c3036;--link:#8ab4f8;--card:#1a1d21;--code-bg:#1e2227;--easy:#4cc38a;--medium:#f0a843;--hard:#ff7b72;--accent:#e8c170}}
*{box-sizing:border-box}
html{font-size:16px}
body{margin:0;background:var(--bg);color:var(--fg);font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif;line-height:1.6}
.wrap{max-width:860px;margin:0 auto;padding:0 16px}
a{color:var(--link);text-decoration:none}a:hover{text-decoration:underline}
.site-header{border-bottom:1px solid var(--line);background:var(--card)}
.site-header .wrap{display:flex;align-items:center;justify-content:space-between;height:56px;gap:16px}
.brand{font-weight:700;color:var(--fg);white-space:nowrap}
.site-header nav{display:flex;gap:16px;font-size:.95rem}
main{padding:24px 16px 48px}
.site-footer{color:var(--muted);font-size:.85rem;border-top:1px solid var(--line);padding:16px}
h1{font-size:1.9rem;line-height:1.25;margin:.2em 0 .4em}
h2{font-size:1.35rem;margin-top:2em;padding-top:.4em;border-top:1px solid var(--line)}
h3{font-size:1.1rem;margin-top:1.6em}
.crumbs,.meta,.links{font-size:.9rem;color:var(--muted)}
.links a{margin-right:14px}
.badge{display:inline-block;padding:1px 9px;border-radius:999px;font-size:.78rem;font-weight:600;color:#fff;vertical-align:middle}
.badge-easy{background:var(--easy)}.badge-medium{background:var(--medium)}.badge-hard{background:var(--hard)}
.pill{display:inline-block;padding:1px 8px;border:1px solid var(--line);border-radius:999px;font-size:.78rem;margin-left:4px;vertical-align:middle}
pre,code{font-family:ui-monospace,SFMono-Regular,Menlo,Consolas,monospace;font-size:.88em}
code{background:var(--code-bg);padding:.1em .35em;border-radius:4px}
pre{background:var(--code-bg);padding:14px 16px;border-radius:8px;overflow:auto;line-height:1.5;border:1px solid var(--line)}
pre code{background:none;padding:0;font-size:1em}
table{border-collapse:collapse;width:100%;margin:1em 0;font-size:.95rem}
th,td{border:1px solid var(--line);padding:6px 10px;text-align:left;vertical-align:top}
th{background:var(--code-bg)}
blockquote{margin:1em 0;padding:.5em 1em;border-left:4px solid var(--accent);background:var(--code-bg);color:var(--fg)}
.pager{display:flex;justify-content:space-between;gap:16px;margin-top:3em;padding-top:1em;border-top:1px solid var(--line);font-size:.95rem}
.topic-list{list-style:none;padding:0;margin:0;display:grid;grid-template-columns:repeat(auto-fill,minmax(260px,1fr));gap:12px}
.topic-list li{background:var(--card);border:1px solid var(--line);border-radius:10px;padding:12px 14px}
.topic-list .count{display:block;color:var(--muted);font-size:.85rem}
.prob-table td:first-child{white-space:nowrap;color:var(--muted)}
.todo{opacity:.55}
.progress{height:8px;background:var(--code-bg);border-radius:999px;overflow:hidden;margin:.4em 0 1.2em}
.progress span{display:block;height:100%;background:var(--easy)}
/* Rouge syntax highlighting (light + dark via variables) */
.highlight .c,.highlight .c1,.highlight .cm,.highlight .cp{color:#6a737d;font-style:italic}
.highlight .k,.highlight .kd,.highlight .kt,.highlight .kc,.highlight .kr{color:#a626a4}
.highlight .s,.highlight .s1,.highlight .s2,.highlight .sc{color:#50a14f}
.highlight .mi,.highlight .mf,.highlight .mh,.highlight .m{color:#986801}
.highlight .nc,.highlight .nn{color:#c18401}
.highlight .nf,.highlight .fm{color:#4078f2}
.highlight .na{color:#e45649}
.highlight .o{color:#0184bc}
@media (prefers-color-scheme:dark){
.highlight .k,.highlight .kd,.highlight .kt,.highlight .kc,.highlight .kr{color:#ff7b72}
.highlight .s,.highlight .s1,.highlight .s2,.highlight .sc{color:#a5d6ff}
.highlight .mi,.highlight .mf,.highlight .mh,.highlight .m{color:#79c0ff}
.highlight .nc,.highlight .nn{color:#ffa657}
.highlight .nf,.highlight .fm{color:#d2a8ff}
.highlight .na{color:#7ee787}
.highlight .o{color:#ff7b72}
.highlight .c,.highlight .c1,.highlight .cm,.highlight .cp{color:#8b949e}}
/* article actions: jump to optimal code */
:root{--btn-bg:#0b57d0;--btn-fg:#ffffff;--ok:#1a7f37}
@media (prefers-color-scheme:dark){:root{--btn-bg:#8ab4f8;--btn-fg:#121417;--ok:#4cc38a}}
.actions{display:flex;flex-wrap:wrap;align-items:center;gap:8px;margin:.7em 0 0}
.actions[hidden]{display:none}
.actions-label{font-size:.88rem;color:var(--muted);margin-right:2px}
.jump{font:inherit;font-size:.9rem;font-weight:600;padding:6px 14px;border:1px solid var(--btn-bg);border-radius:8px;background:transparent;color:var(--btn-bg);cursor:pointer}
.jump[data-jump="optimal"]{background:var(--btn-bg);color:var(--btn-fg)}
.jump:hover:not(:disabled){filter:brightness(1.12)}
.jump:disabled{opacity:.4;cursor:not-allowed}
.jump:focus-visible,.copy-btn:focus-visible{outline:2px solid var(--btn-bg);outline-offset:2px}
/* code block toolbar with copy button */
.code-bar{display:flex;justify-content:space-between;align-items:center;gap:8px;font-size:.78rem;color:var(--muted);background:var(--code-bg);border:1px solid var(--line);border-bottom:0;border-radius:8px 8px 0 0;padding:4px 6px 4px 12px}
.has-bar pre{margin-top:0;border-top-left-radius:0;border-top-right-radius:0}
.highlighter-rouge{margin:1em 0;scroll-margin-top:12px}
.highlighter-rouge pre{margin-bottom:0}
.copy-btn{font:inherit;font-size:.78rem;line-height:1.4;padding:2px 10px;border:1px solid var(--line);border-radius:6px;background:var(--card);color:var(--fg);cursor:pointer;min-width:64px}
.copy-btn:hover{border-color:var(--muted)}
.copy-btn.copied{color:var(--ok);border-color:var(--ok)}
@keyframes flash{0%{box-shadow:0 0 0 3px var(--btn-bg)}100%{box-shadow:0 0 0 3px transparent}}
.flash{animation:flash 1.6s ease-out;border-radius:8px}
/* problem search */
.search{margin:1.2em 0 .6em}
.search input{width:100%;font:inherit;font-size:1rem;padding:10px 14px;border:1px solid var(--line);border-radius:10px;background:var(--card);color:var(--fg)}
.search input::placeholder{color:var(--muted)}
.search input:focus{outline:2px solid var(--btn-bg);outline-offset:1px;border-color:transparent}
.search-status{margin:.4em 2px 0;font-size:.85rem;color:var(--muted);min-height:1.2em}
.search-results{list-style:none;padding:0;margin:.4em 0 1.6em;border:1px solid var(--line);border-radius:10px;overflow:hidden;background:var(--card)}
.search-results li{display:flex;flex-wrap:wrap;align-items:baseline;gap:4px 10px;padding:9px 14px;border-top:1px solid var(--line)}
.search-results li:first-child{border-top:0}
.result-title{font-weight:600}
.result-meta{flex-basis:100%;font-size:.82rem;color:var(--muted)}
.search-empty{color:var(--muted);font-size:.92rem}
.prob-table tr[hidden],.prob-table[hidden],h2[hidden]{display:none}
/* header on narrow screens: keep nav links on one line each and let the bar wrap */
.site-header nav a{white-space:nowrap}
@media (max-width:520px){
  .site-header .wrap{height:auto;flex-wrap:wrap;row-gap:4px;padding-top:10px;padding-bottom:10px}
  .site-header nav{gap:14px;font-size:.9rem}
}
/* problem tables scroll inside themselves on narrow screens instead of widening the page */
@media (max-width:700px){
  .prob-table{display:block;overflow-x:auto;-webkit-overflow-scrolling:touch}
  .prob-table td:nth-child(2){min-width:160px}
}
@media (max-width:520px){
  .actions-label{flex-basis:100%}
  .jump{flex:1 1 0;padding:7px 6px}
}

/* previous / next links: bottom pager plus inline copies under Jump to code and after solutions */
.pager a{max-width:48%}
.pager .next{margin-left:auto;text-align:right}
.pager-inline{margin:1em 0 1.4em;padding:.55em 0;border-top:1px solid var(--line);border-bottom:1px solid var(--line);font-size:.9rem}