Added 1 JS solution for problem 417

Author: imaginate

javascript/417-Pacific-Atlantic-Water-Flow.js

@@ -0,0 +1,95 @@
+//////////////////////////////////////////////////////////////////////////////
+// Depth First Search (DFS)
+// Time: O(m*n)
+// Space: O(m*n)
+// You can implement either Depth First Search (DFS) or Breadth First Search
+// (BFS). The only noticeable impact is the performance cost of the BFS queue
+// is higher than the DFS call stack. Also note if you are implementing BFS
+// that you should terminate the search at the dequeue call if the coordinates
+// signify that the cell has already been traversed as well as terminating the
+// search before the enqueue call at each of the four supplemental directions.
+//////////////////////////////////////////////////////////////////////////////
+
+class Drains {
+    constructor(west = false, east = false) {
+        this.west = west;
+        this.east = east;
+    }
+}
+
+/**
+ * @param {number[][]} heights
+ * @return {number[][]}
+ */
+function pacificAtlantic(heights) {
+    
+    const rowLen = heights.length;
+    const colLen = heights[0].length;
+    const lastRow = rowLen - 1;
+    const lastCol = colLen - 1;
+    const drainage = new Array(rowLen);
+    const drainsBoth = [];
+    
+    for (let r = 0; r < rowLen; ++r) {
+        drainage[r] = new Array(colLen);
+        for (let c = 0; c < colLen; ++c) {
+            drainage[r][c] = new Drains();
+        }
+    }
+    
+    for (let r = 0; r < rowLen; ++r) {
+        markDrainage(r, 0, true);
+        markDrainage(r, lastCol);
+    }
+    for (let c = 0; c < colLen; ++c) {
+        markDrainage(0, c, true);
+        markDrainage(lastRow, c);
+    }
+    
+    return drainsBoth;
+    
+    /**
+     * @param {number} r
+     * @param {number} c
+     * @param {boolean=} west = `false`
+     * @param {number=} prev = `-Infinity`
+     * @return {void}
+     */
+    function markDrainage(r, c, west = false, prev = -Infinity) {
+        
+        if (!inBounds(r, c)
+            || heights[r][c] < prev
+            || (west && drainage[r][c].west)
+            || (!west && drainage[r][c].east)
+        ) {
+            return;
+        }
+        
+        const drains = drainage[r][c];
+        const height = heights[r][c];
+        
+        if (west) {
+            drains.west = true;
+        } else {
+            drains.east = true;
+        }
+        
+        if (drains.west && drains.east) {
+            drainsBoth.push([ r, c ]);
+        }
+        
+        markDrainage(r - 1, c, west, height);
+        markDrainage(r + 1, c, west, height);
+        markDrainage(r, c - 1, west, height);
+        markDrainage(r, c + 1, west, height);
+    }
+    
+    /**
+     * @param {number} r
+     * @param {number} c
+     * @return {boolean}
+     */
+    function inBounds(r, c) {
+        return r >= 0 && c >= 0 && r < rowLen && c < colLen;
+    }
+}