110. Balanced Binary Tree 解题记录

题目描述：

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.

 

检测一个二叉树是不是二叉平衡树，二叉平衡树的性质是：每个分支的深度相差不超过1。

解题思路：

这题可以用递归求解：返回节点两个分支的深度，比较这两个分支深度相差是否大于1，是的话返回-1，不是则返回最大深度（若两个分支深度中有-1，直接返回-1）。代码基本和求二叉树最大深度一样，只是多了一道比较深度的步骤。

代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int checkBalanced(TreeNode* root){
        if(!root)
            return 0;
        int left=checkBalanced(root->left),right=checkBalanced(root->right);
        if(left==-1||right==-1||abs(right-left)>1)
            return -1;
        return max(left,right)+1;
    }
    bool isBalanced(TreeNode* root) {
        if(checkBalanced(root)==-1)
            return false;
        else
            return true;
    }
};