problem_247.js

/**
 * Definition for a binary tree node.
 */
function TreeNode(val) {
  this.val = val;
  this.left = null;
  this.right = null;
}

/**
 * Given a binary tree, determine whether or not it is height-balanced.
 * Time Complexity: O(n)
 * @param {TreeNode} root
 * @return {boolean}
 */
function isHeightBalanced(root) {
  const dfs = node => {
    if (!node) return 0;
    const left = 1 + dfs(node.left);
    const right = 1 + dfs(node.right);
    if (Math.abs(left - right) > 1) return -1;
    return Math.max(left, right);
  };

  return dfs(root) !== -1;
}

const root = new TreeNode(3);
const a = new TreeNode(9);
const b = new TreeNode(20);
const c = new TreeNode(15);
const d = new TreeNode(7);

root.left = a;
root.right = b;
b.left = c;
b.right = d;

//   3
//  / \
// 9  20
//   /  \
//  15   7

console.log(isHeightBalanced(root)); // true

const e = new TreeNode(30);
d.right = e;

//   3
//  / \
// 9  20
//   /  \
//  15   7
//        \
//         30

console.log(isHeightBalanced(root)); // false