/**
 * 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) {
        return dfs(root) != -1;
    }
    int dfs(TreeNode* root) {
        if (root == NULL) return 0;
        int left = dfs(root->left);
        if (left == -1) return -1;
        int right = dfs(root->right);
        if (right == -1) return -1;
        
        if (abs(left - right) > 1) return -1;
        return max(left, right) + 1;
    }
};