class Solution {

public:

    // 返回以该节点为根节点的二叉树的高度，如果不是平衡二叉树了则返回 - 1

    int getHeight(TreeNode* node) {

        if (node == NULL) {

            return 0;

        }

        int leftHeight = getHeight(node->left);

        if (leftHeight == -1) return -1;

        int rightHeight = getHeight(node->right);

        if (rightHeight == -1) return -1;

        return abs(leftHeight - rightHeight) > 1 ? -1 : 1 + max(leftHeight, rightHeight);

    }

    bool isBalanced(TreeNode* root) {

        return getHeight(root) == -1 ? false : true;

    }

};