Add solution #827

Author: JoshCrozier

README.md

@@ -311,6 +311,7 @@
 819|[Most Common Word](./0819-most-common-word.js)|Easy|
 821|[Shortest Distance to a Character](./0821-shortest-distance-to-a-character.js)|Easy|
 824|[Goat Latin](./0824-goat-latin.js)|Easy|
+827|[Making A Large Island](./0827-making-a-large-island.js)|Hard|
 831|[Masking Personal Information](./0831-masking-personal-information.js)|Medium|
 841|[Keys and Rooms](./0841-keys-and-rooms.js)|Medium|
 844|[Backspace String Compare](./0844-backspace-string-compare.js)|Easy|

solutions/0827-making-a-large-island.js

@@ -0,0 +1,65 @@
+/**
+ * 827. Making A Large Island
+ * https://leetcode.com/problems/making-a-large-island/
+ * Difficulty: Hard
+ *
+ * You are given an n x n binary matrix grid. You are allowed to change at most one 0 to be 1.
+ *
+ * Return the size of the largest island in grid after applying this operation.
+ *
+ * An island is a 4-directionally connected group of 1s.
+ */
+
+/**
+ * @param {number[][]} grid
+ * @return {number}
+ */
+var largestIsland = function(grid) {
+  function traverse(tile, grid, i, j) {
+    if (i < 0 || j < 0 || i >= grid.length || j >= grid[0].length
+        || grid[i][j] === 0 || grid[i][j] === tile) {
+      return 0;
+    }
+    grid[i][j] = tile;
+    return 1 + (
+      traverse(tile, grid, i + 1, j)
+      + traverse(tile, grid, i - 1, j)
+      + traverse(tile, grid, i, j + 1)
+      + traverse(tile, grid, i, j - 1)
+    );
+  };
+
+  const map = new Map();
+  let tile = 2;
+  let result = -1;
+
+  for (let i = 0; i < grid.length; i++) {
+    for (let j = 0; j < grid.length; j++) {
+      if (grid[i][j] === 1) {
+        const value = traverse(tile, grid, i, j);
+        map.set(tile, value);
+        tile += 1;
+        result = Math.max(result, value);
+      }
+    }
+  }
+
+  map.set(0, 0);
+
+  for (let i = 0; i < grid.length; i++) {
+    for (let j = 0; j < grid.length; j++) {
+      if (!grid[i][j]) {
+        const seen = new Set();
+        let sum = 0;
+        if (i > 0) seen.add(grid[i - 1][j]);
+        if (j > 0) seen.add(grid[i][j - 1]);
+        if (i < grid.length - 1) seen.add(grid[i + 1][j]);
+        if (j < grid.length - 1) seen.add(grid[i][j + 1]);
+        seen.forEach(val => sum += map.get(val));
+        result = Math.max(result, sum + 1);
+      }
+    }
+  }
+
+  return result;
+};