/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    int dfs_height(TreeNode* root)
    {
        if (root == nullptr)
            return 0;

        int left_height = dfs_height(root->left);
        

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

        int right_height = dfs_height(root->right);
        

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

        if (abs(left_height - right_height) > 1)
            return -1;

        return max(left_height, right_height) + 1;
    }

public:
    bool isBalanced(TreeNode* root)
    {
        return dfs_height(root) != -1;
    }
};