Balanced Binary Tree -- leetcode

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.

基本思路：

判断二叉树是否平衡，判断其左右子树深度差距总不超过1.

引入一辅助函数，其返回树的深度。

同时，如果其发现自己左右子树已经不平稀，则返回-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 {public:    bool isBalanced(TreeNode* root) {        return helper(root) != -1;    }        int helper(TreeNode *root) {        if (!root) return 0;        const int left = helper(root->left);        if (left == -1)             return left;        const int right = helper(root->right);        if (right == -1 || abs(left-right)>1)             return -1;        return max(left, right) + 1;    }};