Added balanced binary tree problem.

README.md

@@ -26,6 +26,7 @@ A collection of JavaScript problems and solutions for studying algorithms.
 - [Construct Binary Tree From Preorder and Inorder Traversal](src/tree/construct-binary-tree-from-preorder-and-inorder-traversal.js)
 - [Construct Binary Tree From Postorder and Inorder Traversal](src/tree/construct-binary-tree-from-inorder-and-postorder-traversal.js)
 - [Convert Sorted Array to Binary Search Tree](src/tree/convert-sorted-array-to-binary-search-tree.js)
+- [Balanced Binary Tree](src/tree/balanced-binary-tree.js)
 
 ### Dynamic Programming
 

src/tree/__tests__/balanced-binary-tree-test.js

@@ -0,0 +1,24 @@
+import { assert } from 'chai';
+import { deserializeTree } from 'utils/tree-util';
+import isBalanced from '../balanced-binary-tree';
+
+describe('Balanced Binary Tree', () => {
+  const testCases = [
+    ['', true],
+    ['1', true],
+    ['1,2', true],
+    ['1,2,null,3', false],
+    ['1,null,2,null,3', false],
+    ['1,2,null,3,null,4', false],
+    ['1,null,2,null,3,null,4', false],
+  ];
+
+  testCases.map((testCase, index) => {
+    it(`should validate tree ${testCase[0]} if it's balanced`, () => {
+      const root = deserializeTree(testCase[0]);
+      const actual = isBalanced(root);
+      const expected = testCase[1];
+      assert.equal(actual, expected);
+    });
+  });
+});

src/tree/balanced-binary-tree.js

@@ -0,0 +1,34 @@
+/**
+ * Given a binary tree, determine if it is height-balanced.
+ * 
+ * For this problem, a height-balanced binary tree is defined as a binary tree
+ * in which the depth of the two subtrees of every node never differ by more than 1.
+ */
+
+const isBalanced = root => {
+  const getHeight = root => {
+    if (!root) {
+      return 0;
+    }
+
+    const leftHeight = getHeight(root.left);
+    if (leftHeight < 0) {
+      return -1;
+    }
+
+    const rightHeight = getHeight(root.right);
+    if (rightHeight < 0) {
+      return -1;
+    }
+
+    if (Math.abs(leftHeight - rightHeight) > 1) {
+      return -1;
+    }
+
+    return 1 + Math.max(leftHeight, rightHeight);
+  };
+
+  return getHeight(root) >= 0;
+};
+
+export default isBalanced;