Given a binary tree, determine if it is AVL balanced tree. http://www.leetcode.com/onlinejudge

leetcode_balanced_binary_tree.cpp

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void balance_depth(TreeNode *root, bool &ret, int &depth) {
        if (root == NULL) {
            ret = true;
            depth = 0;
            return;
        }
        bool left_ret = true, right_ret = true;
        int left_depth = 0, right_depth = 0;
        balance_depth(root->left, left_ret, left_depth);
        balance_depth(root->right, right_ret, right_depth);
        ret = left_ret & right_ret & (abs(left_depth - right_depth) <= 1);
        depth = 1 + max(left_depth, right_depth);
    }

    bool isBalanced(TreeNode *root) {
        if (root == NULL) {
            return true;
        }
        bool ret = true;
        int depth = 0;
        balance_depth(root, ret, depth);
        return ret;
    }
};