[reeseo3o] WEEK 09 Solutions

linked-list-cycle/reeseo3o.js

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+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+const hasCycle = (head) => {
+  if (head === null || head.next === null) {
+    return false;
+  }
+
+  let slow = head;
+  let fast = head.next;
+
+  while (slow !== fast) {
+    if (fast === null || fast.next === null) {
+      return false;
+    }
+    slow = slow.next;
+    fast = fast.next.next;
+  }
+
+  return true;
+};
+

pacific-atlantic-water-flow/reeseo3o.js

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+// Time Complexity: O(m * n)
+// Space Complexity: O(m * n)
+
+const pacificAtlantic = (heights) => {
+  if (!heights?.length || !heights[0]?.length) {
+    return [];
+  }
+
+  const m = heights.length;
+  const n = heights[0].length;
+  const pacific = Array.from({ length: m }, () => Array(n).fill(false));
+  const atlantic = Array.from({ length: m }, () => Array(n).fill(false));
+  const dirs = [
+    [-1, 0],
+    [1, 0],
+    [0, -1],
+    [0, 1],
+  ];
+
+  const dfs = (visited, r, c) => {
+    visited[r][c] = true;
+    for (const [dr, dc] of dirs) {
+      const nr = r + dr;
+      const nc = c + dc;
+      if (nr < 0 || nr >= m || nc < 0 || nc >= n || visited[nr][nc]) continue;
+      if (heights[nr][nc] < heights[r][c]) continue;
+      dfs(visited, nr, nc);
+    }
+  };
+
+  for (let i = 0; i < m; i++) {
+    dfs(pacific, i, 0);
+    dfs(atlantic, i, n - 1);
+  }
+  for (let j = 0; j < n; j++) {
+    dfs(pacific, 0, j);
+    dfs(atlantic, m - 1, j);
+  }
+
+  const result = [];
+  for (let r = 0; r < m; r++) {
+    for (let c = 0; c < n; c++) {
+      if (pacific[r][c] && atlantic[r][c]) {
+        result.push([r, c]);
+      }
+    }
+  }
+  return result;
+};