【Leetcode】110. Balanced Binary Tree（平衡二叉树）

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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.

题目大意：

判断给出的二叉树是否为平衡二叉树。

平衡二叉树的定义：它是一棵空树或它的左右两个子树的高度差的绝对值不超过1，并且左右两个子树都是一棵平衡二叉树。 

解题思路：

递归求解，判断左右两边的最大深度差值是否大于1。 

/**
 * 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 {
private:
    int helper(TreeNode *root){
        
        if(root==NULL){
            return 0;
        }
        
        int l = helper(root->left);
        int r = helper(root->right);
        
        if (l == -1 or r==-1 or abs(l-r)>1){
            return -1;
        }
        
        return max(l,r)+1;
    }
public:
    bool isBalanced(TreeNode* root) {
        
        return helper(root)!=-1;
        
    }
};