docs(docs): add new problem documentation pages and source selector component

apps/docs/blume.config.ts

@@ -142,11 +142,8 @@ export default defineConfig({
     sidebar: {
       display: "flat", // "flat" | "group" | "page"
     },
-    // The glossary is no content folder any more, so it is pinned above the
+    // No `featured`: Blume renders it above the `Sidebar` slot, and the
-    // sidebar tree, on every page and breakpoint.
+    // content-source selector must sit above the pinned links, so
-    featured: [
+    // components/SourceSidebar.astro renders both (Changelog, Appendix).
-      { label: "Changelog", href: "/changelog", icon: "clock" },
-      { label: "Appendix", href: "/appendix", icon: "library" },
-    ],
   },
 });

apps/docs/components.ts

@@ -16,5 +16,8 @@ export default defineComponents({
   layout: {
     // Topic landing pages' "what links here": every problem pointing at the page.
     PageFooter: "./components/BacklinksList.astro",
+    // Content-source selector + pinned links above the nav tree, scoped to the
+    // active source (sources.ts).
+    Sidebar: "./components/SourceSidebar.astro",
   },
 });

apps/docs/components/SourceSidebar.astro

@@ -0,0 +1,117 @@
+---
+/**
+ * The `Sidebar` slot (also the mobile drawer's nav, which defaults to it): a
+ * Fumadocs-style content-source selector, then the pinned links, then
+ * Blume's own nav tree scoped to the active source (`sources.ts`).
+ *
+ * The pinned links live here, not in `navigation.featured`: Blume renders
+ * featured links above this slot, so the selector could not sit over them.
+ *
+ * The selector is a `<details>` with Blume's `data-blume-dropdown` light
+ * dismiss (outside press, Escape, focus leaving), like its own NavSelector.
+ */
+import Icon from "blume/components/Icon.astro";
+import { withBase, withMountedBase } from "blume/components/islands/base-path.ts";
+import NavTree from "blume/components/layout/NavTree.astro";
+import type { UIStrings } from "blume/core/i18n-ui.ts";
+import { isUnderPath } from "blume/core/navigation.ts";
+import type { NavNode } from "blume/core/types.ts";
+import { SOURCES } from "../sources.ts";
+
+interface Props {
+  items: NavNode[];
+  currentRoute: string;
+  strings?: UIStrings["nav"];
+  ids?: Map<NavNode, string>;
+  fragmentBase?: string;
+}
+
+const PINNED = [
+  { label: "Changelog", href: "/changelog", icon: "clock" },
+  { label: "Appendix", href: "/appendix", icon: "library" },
+];
+
+const { items, currentRoute, ...tree } = Astro.props;
+
+const active =
+  SOURCES.filter((s) => isUnderPath(currentRoute, s.path)).sort(
+    (a, b) => b.path.length - a.path.length,
+  )[0] ?? SOURCES[0]!;
+
+const claims = (node: NavNode, path: string) => {
+  const at = node.kind === "page" ? node.route : node.path;
+  return at !== undefined && isUnderPath(at, path);
+};
+
+// The root source keeps every node no other source claims; any other source
+// shows its folder's children, as if that folder were the content root.
+const scoped =
+  active.path === "/"
+    ? items.filter(
+        (node) => !SOURCES.some((s) => s.path !== "/" && claims(node, s.path)),
+      )
+    : items.flatMap((node) =>
+        node.kind === "group" && node.path === active.path ? node.children : [],
+      );
+---
+
+<details class="group relative mb-3" data-blume-dropdown>
+  <summary
+    aria-label={`Content source: ${active.label}`}
+    class="flex w-full cursor-pointer list-none items-center gap-2.5 rounded-xl border border-border bg-muted/50 p-1.5 pe-2.5 text-foreground text-sm transition-colors hover:bg-muted group-open:bg-muted [&::-webkit-details-marker]:hidden"
+  >
+    <span class="flex size-7 shrink-0 items-center justify-center rounded-lg border border-border bg-background">
+      <Icon name={active.icon} size={15} />
+    </span>
+    <span class="min-w-0 flex-1 truncate font-medium">{active.label}</span>
+    <Icon class="shrink-0 text-muted-foreground" name="chevrons-up-down" size={14} />
+  </summary>
+  <div
+    class="absolute inset-x-0 z-50 mt-1.5 rounded-xl border border-border bg-background p-1 shadow-xl"
+    data-blume-dropdown-panel
+  >
+    {
+      SOURCES.map((source) => (
+        <a
+          aria-current={source === active ? "true" : undefined}
+          class="flex items-start gap-2.5 rounded-lg p-2 text-muted-foreground text-sm transition-colors hover:bg-muted hover:text-foreground aria-[current=true]:text-foreground"
+          href={withMountedBase(source.path)}
+        >
+          <Icon class="mt-0.5 shrink-0" name={source.icon} size={16} />
+          <span class="min-w-0 flex-1">
+            <span class="block font-medium">{source.label}</span>
+            <span class="block text-muted-foreground text-xs">{source.description}</span>
+          </span>
+          {source === active && <Icon class="mt-0.5 shrink-0" name="check" size={14} />}
+        </a>
+      ))
+    }
+  </div>
+</details>
+
+<nav aria-label="Featured" class="mb-4 border-border border-b pb-4">
+  <ul class="m-0 list-none p-0">
+    {
+      PINNED.map((link) => (
+        <li>
+          <a
+            aria-current={currentRoute === link.href ? "page" : undefined}
+            class="flex items-center gap-2 rounded-[0.65rem] px-2.5 py-1.5 text-muted-foreground text-sm transition-colors hover:bg-muted hover:text-foreground aria-[current=page]:bg-muted aria-[current=page]:text-foreground"
+            href={withBase(link.href)}
+          >
+            <Icon class="shrink-0" name={link.icon} size={14} />
+            <span class="flex-1 truncate">{link.label}</span>
+          </a>
+        </li>
+      ))
+    }
+  </ul>
+</nav>
+
+<NavTree {...tree} currentRoute={currentRoute} items={scoped} />
+
+<script>
+  import { installDropdownDismiss } from "blume/components/dropdown-dismiss.ts";
+
+  installDropdownDismiss();
+</script>

apps/docs/content/(01-linear-structures)/monotonic-stack/901_online-stock-span.mdx

@@ -0,0 +1,45 @@
+---
+title: '901. Online Stock Span'
+description: Design an algorithm that collects daily price quotes for some stock and returns the span of that stock's price for the current day
+icon: dot
+topics:
+  - { name: "Stack", slug: "stack" }
+  - { name: "Design", slug: "design" }
+  - { name: "Monotonic Stack", slug: "monotonic-stack" }
+  - { name: "Data Stream", slug: "data-stream" }
+issue: "https://github.com/prdlk/leetcode/issues/75"
+sidebar:
+  label: 'Online Stock Span'
+  badge: 'Medium'
+---
+
+### Example 1:
+- Input: ``
+- Output: ``
+- Explanation: Input ["StockSpanner", "next", "next", "next", "next", "next", "next", "next"] `[[]`, `[100]`, `[80]`, `[60]`, `[70]`, `[60]`, `[75]`, [85]] Output [null, `1`, `1`, `1`, `2`, `1`, `4`, 6] Explanation StockSpanner stockSpanner = new StockSpanner(); stockSpanner.next(100); // return `1` stockSpanner.next(80);  // return `1` stockSpanner.next(60);  // return `1` stockSpanner.next(70);  // return `2` stockSpanner.next(60);  // return `1` stockSpanner.next(75);  // return `4`, because the last `4` prices (including today's `price` of `75`) were less than or equal to today's `price`. stockSpanner.next(85);  // return `6`
+
+### Constraints:
+
+- `1 <= price <= 10^5`
+- At most 10^4 calls will be made to next.
+
+## Solution
+
+```py
+class StockSpanner:
+    def __init__(self):
+        self.stack = []
+
+    def next(self, price: int) -> int:
+        ans = 1
+        while self.stack and self.stack[-1][0] <= price:
+            ans += self.stack.pop()[1]
+
+        self.stack.append([price, ans])
+        return ans
+
+
+# Your StockSpanner object will be instantiated and called as such:
+# obj = StockSpanner()
+# param_1 = obj.next(price)
+```

apps/docs/content/(01-linear-structures)/sliding-window/76_minimum-window-substring.mdx

@@ -0,0 +1,46 @@
+---
+title: '76. Minimum Window Substring'
+description: Given two strings s and t of lengths m and n respectively, return the minimum window substring of s such that every character in t (including duplicates) is included in the window. If there is no such substring, return the empty string ""
+icon: dot
+topics:
+  - { name: "Hash Table", slug: "hash-table" }
+  - { name: "String", slug: "string" }
+  - { name: "Sliding Window", slug: "sliding-window" }
+issue: "https://github.com/prdlk/leetcode/issues/57"
+sidebar:
+  label: 'Minimum Window Substring'
+  badge: 'Hard'
+---
+
+::::warning
+Could you find an algorithm that runs in O(m + n) time?
+::::
+
+### Example 1:
+- Input: `s = "ADOBECODEBANC", t = "ABC"`
+- Output: `"BANC"`
+- Explanation: The minimum window substring "BANC" includes 'A', 'B', and 'C' from string `t`.
+
+### Example 2:
+- Input: `s = "a", t = "a"`
+- Output: `"a"`
+- Explanation: The entire string `s` is the minimum window.
+
+### Example 3:
+- Input: `s = "a", t = "aa"`
+- Output: `""`
+- Explanation: Both 'a's from `t` must be included in the window. Since the largest window of `s` only has one 'a', return empty string.
+
+### Constraints:
+
+- `m == s.length`
+- `n == t.length`
+- `1 <= m, n <= 10^5`
+- `s` and `t` consist of uppercase and lowercase English letters.
+
+## Solution
+
+```py
+class Solution:
+    def minWindow(self, s: str, t: str) -> str:
+```

apps/docs/content/(01-linear-structures)/stack/224_basic-calculator.mdx

@@ -0,0 +1,43 @@
+---
+title: '224. Basic Calculator'
+description: Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation
+icon: dot
+topics:
+  - { name: "Math", slug: "math" }
+  - { name: "String", slug: "string" }
+  - { name: "Stack", slug: "stack" }
+  - { name: "Recursion", slug: "recursion" }
+issue: "https://github.com/prdlk/leetcode/issues/71"
+sidebar:
+  label: 'Basic Calculator'
+  badge: 'Hard'
+---
+
+### Example 1:
+- Input: `s = "1 + 1"`
+- Output: `2`
+
+### Example 2:
+- Input: `s = " 2-1 + 2 "`
+- Output: `3`
+
+### Example 3:
+- Input: `s = "(1+(4+5+2)-3)+(6+8)"`
+- Output: `23`
+
+### Constraints:
+
+- `1 <= s.length <= 3 * 10^5`
+- `s` consists of digits, '+', '-', '(', ')', and ' '.
+- `s` represents a valid expression.
+- '+' is not used as a unary operation (i.e., "+1" and "+(2 + 3)" is invalid).
+- '-' could be used as a unary operation (i.e., "-1" and "-(2 + 3)" is valid).
+- There will be no two consecutive operators in the input.
+- Every number and running calculation will fit in a signed 32-bit integer.
+
+## Solution
+
+```py
+class Solution:
+    def calculate(self, s: str) -> int:
+```

apps/docs/content/system-design/index.mdx

@@ -0,0 +1,27 @@
+---
+title: System Design
+description: Notes for system design interviews — how to scope a problem, size it, and pick the parts that carry the load.
+icon: network
+sidebar:
+  label: Overview
+---
+
+## How an interview runs
+
+<Steps>
+  <Step title="Requirements">
+    Pin down the functional requirements (what the system does) and the non-functional ones (latency, availability, consistency, scale) before drawing anything.
+  </Step>
+  <Step title="Estimates">
+    Size the traffic, storage and bandwidth with back-of-the-envelope numbers, so later choices have a reason.
+  </Step>
+  <Step title="API and data model">
+    Define the endpoints and the entities they read and write.
+  </Step>
+  <Step title="High-level design">
+    Draw the request path end to end: clients, load balancers, services, caches, stores and queues.
+  </Step>
+  <Step title="Deep dives">
+    Take the hardest part (a hot key, fan-out, a consistency guarantee) and work through its tradeoffs.
+  </Step>
+</Steps>

apps/docs/redirects.ts

@@ -29,6 +29,7 @@ export default [
   { from: "/71", to: "/stack/simplify-path" },
   { from: "/73", to: "/matrix-index-math/set-matrix-zeroes" },
   { from: "/74", to: "/binary-search/search-a-2d-matrix" },
+  { from: "/76", to: "/sliding-window/minimum-window-substring" },
   { from: "/78", to: "/backtracking/subsets" },
   { from: "/80", to: "/two-pointers/remove-duplicates-from-sorted-array-ii" },
   { from: "/84", to: "/monotonic-stack/largest-rectangle-in-histogram" },
@@ -61,6 +62,7 @@ export default [
   { from: "/211", to: "/trie/design-add-and-search-words-data-structure" },
   { from: "/215", to: "/heap-priority-queue/kth-largest-element-in-an-array" },
   { from: "/217", to: "/hash-table/contains-duplicate" },
+  { from: "/224", to: "/stack/basic-calculator" },
   { from: "/226", to: "/tree-dfs/invert-binary-tree" },
   { from: "/230", to: "/binary-search-tree/kth-smallest-element-in-a-bst" },
   { from: "/235", to: "/binary-search-tree/lowest-common-ancestor-of-a-binary-search-tree" },
@@ -89,6 +91,7 @@ export default [
   { from: "/739", to: "/monotonic-stack/daily-temperatures" },
   { from: "/853", to: "/monotonic-stack/car-fleet" },
   { from: "/875", to: "/binary-search/koko-eating-bananas" },
+  { from: "/901", to: "/monotonic-stack/online-stock-span" },
   { from: "/973", to: "/heap-priority-queue/k-closest-points-to-origin" },
   { from: "/974", to: "/prefix-sum/subarray-sums-divisible-by-k" },
   { from: "/977", to: "/two-pointers/squares-of-a-sorted-array" },

apps/docs/sources.ts

@@ -0,0 +1,34 @@
+/**
+ * The site's content sources — what the sidebar's source selector switches
+ * between (`components/SourceSidebar.astro`), Fumadocs' "root folders". A
+ * source owns every route under its `path`, and the sidebar shows only the
+ * active source's pages. The source at `/` owns whatever no other source
+ * claims, which is how the LeetCode campaign keeps its routes (`/tree-dfs/…`)
+ * while a later source mounts beside it.
+ *
+ * Adding one: a content folder at the path (`content/system-design/`) with an
+ * `index.mdx`, so the selector's link is a page, plus a row here. The active
+ * source is the deepest `path` the current route sits under.
+ */
+export interface Source {
+  label: string;
+  description: string;
+  /** A Blume built-in icon name. */
+  icon: string;
+  path: string;
+}
+
+export const SOURCES: Source[] = [
+  {
+    label: "LeetCode",
+    description: "Data structures & algorithms",
+    icon: "binary",
+    path: "/",
+  },
+  {
+    label: "System Design",
+    description: "Distributed systems interviews",
+    icon: "network",
+    path: "/system-design",
+  },
+];