110. Balanced Binary Tree（平衡树）

110. Balanced Binary Tree

Easy

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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.

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.

方法一：递归

这道题是104题的变式题，再求高度的基础上要求平衡树左右子树高度差都小于等于 1。

时间复杂度：o（n）                     空间复杂度：o（1）