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/**
 * 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 check(root) != -1;
    }
    
    int check(TreeNode* node) {
        if (node == NULL) return 0;
        
        int levl = check(node->left);
        int revl = check(node->right);
        
        if (dis(levl - revl) > 1 || levl == -1 || revl == -1) {
            return -1;
        }
        
        return (levl > revl ? levl : revl) + 1;
    }
    
    int dis(int i) {
        return i < 0 ? -i : i;
    }
    
};