:root{--ink:#172d4e;--muted:#64748b;--line:#dce5ed;--green:#09865e;--pale:#eaf7f1;--code:#f6f8fa}
*{box-sizing:border-box}
body{margin:0;color:var(--ink);background:#fff;font:16px/1.8 -apple-system,BlinkMacSystemFont,"Segoe UI","PingFang SC","Microsoft YaHei",sans-serif}
a{color:var(--green);text-decoration:none}a:hover{text-decoration:underline}
.wrap{max-width:920px;margin:0 auto;padding:0 20px}
.top{border-bottom:1px solid var(--line);background:#fafcfd}
.top .wrap{display:flex;align-items:center;justify-content:space-between;gap:16px;min-height:56px;flex-wrap:wrap}
.brand{font-weight:700;color:var(--ink)}
.top nav{display:flex;gap:18px;font-size:15px}
.crumbs{margin:18px 0 0;font-size:14px;color:var(--muted)}.crumbs i{margin:0 8px;font-style:normal;color:#b6c2cf}
h1{font-size:30px;line-height:1.35;margin:16px 0 10px}
h2{font-size:21px;margin:34px 0 10px;padding-top:6px;border-top:1px solid var(--line)}
h2 small,h3 small{font-weight:400;color:var(--muted);font-size:14px;margin-left:6px}
h3{font-size:17px;margin:20px 0 8px}
.head{margin-bottom:8px}
.tags{display:flex;flex-wrap:wrap;gap:8px;font-size:14px}
.tags>*{border:1px solid var(--line);border-radius:999px;padding:1px 12px}
.d.easy{color:#09865e;border-color:#b9e3d3}.d.mid{color:#a9630a;border-color:#f0d6ac}.d.hard{color:#b64040;border-color:#efc2c2}
.cta{display:flex;flex-wrap:wrap;gap:10px;margin:18px 0 6px}
.btn{display:inline-flex;align-items:center;min-height:42px;padding:8px 16px;border:1px solid #bfcddc;border-radius:6px;color:var(--ink);background:#fff;font-size:15px}
.btn:hover{background:var(--pale);border-color:var(--green);text-decoration:none}
.btn.primary{background:var(--green);border-color:var(--green);color:#fff;font-weight:600}.btn.primary:hover{background:#066b4a}
code{font:14px/1.6 ui-monospace,SFMono-Regular,Menlo,Consolas,monospace;background:var(--code);padding:1px 5px;border-radius:4px}
pre{overflow:auto;background:var(--code);border:1px solid var(--line);border-radius:8px;padding:12px 14px;margin:8px 0;font:14px/1.6 ui-monospace,SFMono-Regular,Menlo,Consolas,monospace;white-space:pre}
pre.ex{white-space:pre-wrap}
pre code{background:none;padding:0}
.explain,.muted{color:var(--muted);font-size:15px}
.exb{margin:12px 0}
.tabs{margin:10px 0}
.tabbar{display:flex;flex-wrap:wrap;gap:4px;border-bottom:1px solid var(--line)}
.tabbar button{font:inherit;font-size:14px;border:0;background:none;padding:6px 12px;cursor:pointer;color:var(--muted);border-bottom:2px solid transparent;margin-bottom:-1px}
.tabbar button[aria-selected=true]{color:var(--green);border-bottom-color:var(--green);font-weight:600}
.tabbar .copy{margin-left:auto;color:var(--green)}
.tabs pre{border-top-left-radius:0;border-top-right-radius:0;margin-top:0}
.alt{margin:18px 0;padding:4px 18px 10px;border:1px solid var(--line);border-radius:10px}
.pn{display:flex;justify-content:space-between;gap:16px;margin:36px 0 0;padding:16px 0;border-top:1px solid var(--line)}
.plist{list-style:none;padding:0;margin:8px 0;columns:2;column-gap:28px}
.plist li{break-inside:avoid;display:flex;gap:8px;align-items:center;padding:5px 0;border-bottom:1px dashed var(--line);font-size:15px}
.plist li a{flex:1}
.plist .t{color:var(--muted);font-size:13px}
.plist .d{font-size:13px;border:0;padding:0}
.topics{display:flex;flex-wrap:wrap;gap:8px;margin:14px 0}
.topics>*{border:1px solid var(--line);border-radius:999px;padding:2px 12px;font-size:14px}
.topics span{background:var(--pale);border-color:var(--green)}
.foot{margin-top:48px;border-top:1px solid var(--line);background:#fafcfd;color:var(--muted);font-size:14px;padding:22px 0}
.foot p{margin:4px 0}
.prep{display:flex;flex-direction:column;gap:4px;margin:30px 0 0;padding:14px 18px;border:1px solid var(--line);border-left:4px solid var(--green);border-radius:8px;background:var(--pale);font-size:15px}
@media (max-width:640px){h1{font-size:24px}.plist{columns:1}.top nav{gap:12px;font-size:14px}}