/**
 * Definition for binary tree
 * 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 || (!root->left&&!root->right)) return true;
        int leftHeight = height(root->left);
        int rightHeight = height(root->right);
        if(abs(leftHeight-rightHeight) > 1)
            return false;
        return isBalanced(root->left) && isBalanced(root->right);
    }
    
    int height(TreeNode *root){
        if(!root)return 0;
        int left = height(root->left);
        int right = height(root->right);
        return left>right?left+1:right+1;
    }
};