Add solution #427

Author: JoshCrozier

README.md

@@ -339,6 +339,7 @@
 421|[Maximum XOR of Two Numbers in an Array](./0421-maximum-xor-of-two-numbers-in-an-array.js)|Medium|
 423|[Reconstruct Original Digits from English](./0423-reconstruct-original-digits-from-english.js)|Medium|
 424|[Longest Repeating Character Replacement](./0424-longest-repeating-character-replacement.js)|Medium|
+427|[Construct Quad Tree](./0427-construct-quad-tree.js)|Medium|
 434|[Number of Segments in a String](./0434-number-of-segments-in-a-string.js)|Easy|
 435|[Non-overlapping Intervals](./0435-non-overlapping-intervals.js)|Medium|
 437|[Path Sum III](./0437-path-sum-iii.js)|Medium|

solutions/0427-construct-quad-tree.js

@@ -0,0 +1,54 @@
+/**
+ * 427. Construct Quad Tree
+ * https://leetcode.com/problems/construct-quad-tree/
+ * Difficulty: Medium
+ *
+ * Given a n * n matrix grid of 0's and 1's only. We want to represent grid with a Quad-Tree.
+ *
+ * Return the root of the Quad-Tree representing grid.
+ *
+ * A Quad-Tree is a tree data structure in which each internal node has exactly four children.
+ * Besides, each node has two attributes:
+ * - val: True if the node represents a grid of 1's or False if the node represents a grid of
+ *   0's. Notice that you can assign the val to True or False when isLeaf is False, and both are
+ *   accepted in the answer.
+ * - isLeaf: True if the node is a leaf node on the tree or False if the node has four children.
+ */
+
+/**
+ * // Definition for a QuadTree node.
+ * function _Node(val,isLeaf,topLeft,topRight,bottomLeft,bottomRight) {
+ *    this.val = val;
+ *    this.isLeaf = isLeaf;
+ *    this.topLeft = topLeft;
+ *    this.topRight = topRight;
+ *    this.bottomLeft = bottomLeft;
+ *    this.bottomRight = bottomRight;
+ * };
+ */
+
+/**
+ * @param {number[][]} grid
+ * @return {_Node}
+ */
+var construct = function(grid) {
+  return build(0, 0, grid.length);
+
+  function build(x, y, n) {
+    const value = grid[x][y];
+    for (let i = x; i < x + n; i++) {
+      for (let j = y; j < y + n; j++) {
+        if (grid[i][j] !== value) {
+          n /= 2;
+          return new _Node(true, false,
+            build(x, y, n),
+            build(x, y + n, n),
+            build(x + n, y, n),
+            build(x + n, y + n, n)
+          );
+        }
+      }
+    }
+    return new _Node(value === 1, true, null, null, null, null);
+  }
+};