leetcode_110. Balanced Binary Tree

題目：

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.

直接的想法就是，比較做右子樹的高度。而這個高度就等於此樹左右子樹最高的樹的高度。所以代碼如下：

/**
 * 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 depth(TreeNode* root){
        if(root == NULL) return 0;
        return 1+max(depth(root->left),depth(root->right));
    }
    bool isBalanced(TreeNode* root) {
        if(root == NULL) return true;
        return abs(depth(root->left) - depth(root->right))<=1 && isBalanced(root->left) && isBalanced(root->right);
    }
};