12.balanced-binary-tree.js

// https://leetcode.com/problems/balanced-binary-tree/
// 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 left and right subtrees of every node differ in height by no more than 1.

// Example 1:
// Input: root = [3,9,20,null,null,15,7]
// Output: true
// Example 2:

// Input: root = [1,2,2,3,3,null,null,4,4]
// Output: false
// Example 3:
// Input: root = []
// Output: true
// Constraints:
// The number of nodes in the tree is in the range [0, 5000].
// -104 <= Node.val <= 104

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */

var isBalanced = function (root) {
  return helper(root, 0) >= 0;
};

var helper = function (root, depth) {
  if (!root) return depth;
  var left = helper(root.left, depth + 1);
  var right = helper(root.right, depth + 1);
  if (left === -1 || right === -1 || Math.abs(left - right) > 1) return -1;
  return Math.max(left, right);
};