LeetCode 0110 - 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 {
private:
    int depth(TreeNode* root) {
        if(root == NULL)
            return 0;
        return max(depth(root->left), depth(root->right)) + 1;
    }
public:
    bool isBalanced(TreeNode* root) {
        if(root == NULL)
            return 1;
        int left = depth(root->left);
        int right = depth(root->right);
        return (abs(left-right) <= 1) && isBalanced(root->left) && isBalanced(root->right);
    }
};