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) return true;
        if(helper(root, 0)==-1) return false;
        else return true;
    }
    int helper(TreeNode* root, int level){
        int l=level,r=level;
        if(root->left) l=helper(root->left, level+1);
        if(root->right) r=helper(root->right, level+1);
        if(l==-1 || r==-1 || abs(l-r)>1) return -1;
        return max(l, r);
    }
};