题目：

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.

提示：

此题要求判断一个给定的二叉树是否是“平衡”的，这里“平衡”的定义是要求每个节点的左右两颗子树高度之差不能大于1，解题的思想是使用DFS。

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:
    bool isBalanced(TreeNode* root) {
        return dfs(root) != -1;
    }
    int dfs(TreeNode* root) {
        if (root == NULL) return 0;
        int left = dfs(root->left);
        if (left == -1) return -1;
        int right = dfs(root->right);
        if (right == -1) return -1;

        if (abs(left - right) > 1) return -1;
        return max(left, right) + 1;
    }
};