class Solution {
  private:
    int treeHeight(TreeNode * root, bool& balanced) {
      if (root == NULL) { balanced = true; return 0; }
      bool left, right;
      int left_height = treeHeight(root->left, left) + 1;
      int right_height = treeHeight(root->right, right) + 1;
      balanced = left && right;
      if (!balanced) return 0;
      int result = left_height - right_height;
      balanced = (result >= -1 && result <= 1);
      return max(left_height, right_height);
    }
  public:
    bool isBalanced(TreeNode *root) {
      if (root == NULL) return true;
      bool left, right;
      int result = treeHeight(root->left, left) - treeHeight(root->right, right);
      return (left&&right)&&(result >= -1 && result <= 1);
    }
};