[LeetCode] Balanced Binary Tree

/**
 * 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) {
        if (root == NULL) return true;
        
        if (Depth(root) == -1) {
            return false;
        } else {
            return true;
        }
    }
    
    int Depth(TreeNode* root) {
        if (root == NULL) return 0;
        
        int left_depth = Depth(root->left);
        int right_depth = Depth(root->right);
        
        if (left_depth == -1 || right_depth == -1 || abs(left_depth - right_depth) > 1) {
            return -1;
        } else {
            return left_depth > right_depth ? left_depth + 1 : right_depth + 1;
        }
    }
};