[LeetCode] 110. Balanced Binary Tree

平衡二叉树。给一个二叉树，请你判断它是否是一个平衡二叉树。平衡二叉树的定义是任何两个叶子节点之间的高度差不能大于1。例子，

Example 1:

Given the following tree [3,9,20,null,null,15,7]:

    3
   / \
  9  20
    /  \
   15   7

Return true.

Example 2:

Given the following tree [1,2,2,3,3,null,null,4,4]:

       1
      / \
     2   2
    / \
   3   3
  / \
 4   4

Return false.

这个题的思路是后序遍历。

时间O(n)

空间O(n)

 1 /**
 2  * @param {TreeNode} root
 3  * @return {boolean}
 4  */
 5 var isBalanced = function (root) {
 6     if (root === null) return true;
 7     return helper(root) !== -1;
 8 };
 9 
10 var helper = function (root) {
11     if (root === null) return 0;
12     let left = helper(root.left);
13     let right = helper(root.right);
14     if (left == -1 || right == -1 || Math.abs(left - right) > 1) {
15         return -1;
16     }
17     return Math.max(left, right) + 1;
18 }