Add week 9 solutions: pacificAtlanticWaterFlow

Author: yolophg

pacific-atlantic-water-flow/yolophg.js

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+// Time Complexity: O(rows + cols)
+// Space Complexity: O(rows + cols)
+
+var pacificAtlantic = function (heights) {
+  const rows = heights.length;
+  const cols = heights[0].length;
+
+  // initialize matrices to keep track of cells reachable by Pacific and Atlantic
+  const pacific = [];
+  const atlantic = [];
+
+  // create 2D arrays filled with false values
+  for (let i = 0; i < rows; i++) {
+    pacific.push(new Array(cols).fill(false));
+    atlantic.push(new Array(cols).fill(false));
+  }
+
+  // directions for moving up, right, down, left
+  const directions = [
+    [0, 1],
+    [1, 0],
+    [0, -1],
+    [-1, 0],
+  ];
+
+  // helper to do DFS
+  function dfs(row, col, ocean) {
+    // mark the cell as reachable
+    ocean[row][col] = true;
+    for (const [dx, dy] of directions) {
+      // check all 4 directions
+      const newRow = row + dx;
+      const newCol = col + dy;
+      // continue if the new cell is within bounds, not visited, and has a height greater than or equal to the current cell
+      if (
+        newRow >= 0 &&
+        newRow < rows &&
+        newCol >= 0 &&
+        newCol < cols &&
+        !ocean[newRow][newCol] &&
+        heights[newRow][newCol] >= heights[row][col]
+      ) {
+        dfs(newRow, newCol, ocean);
+      }
+    }
+  }
+
+  // start DFS from the Pacific ocean borders
+  for (let i = 0; i < rows; i++) {
+    // left edge
+    dfs(i, 0, pacific);
+    // right edge
+    dfs(i, cols - 1, atlantic);
+  }
+
+  // start DFS from the Atlantic ocean borders
+  for (let j = 0; j < cols; j++) {
+    // top edge
+    dfs(0, j, pacific);
+    dfs(rows - 1, j, atlantic); // Bottom edge
+  }
+
+  const result = [];
+  // check each cell can reach both oceans
+  for (let i = 0; i < rows; i++) {
+    for (let j = 0; j < cols; j++) {
+      if (pacific[i][j] && atlantic[i][j]) {
+        // if it can reach both
+        result.push([i, j]);
+      }
+    }
+  }
+
+  // return the final list
+  return result;
+};