Code

/**
 * 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 {
public:
    bool isbalanced = true;
    bool isBalanced(TreeNode* root) {
        if (root == nullptr) return true;
        height(root);
        return isbalanced;
    }

    int height(TreeNode* node) {
        if (!isbalanced) return -1;
        if (node->left == nullptr && node->right == nullptr) return 0;

        int hg = 0;
        if (node->left == nullptr || node->right == nullptr) {
            if (node->left == nullptr) {
                hg = height(node->right) + 1;
                if (hg > 1) {
                    isbalanced = false;
                    return 0;
                }
                return hg;
            }else{
                hg = height(node->left) + 1;
                if (hg > 1) {
                    isbalanced = false;
                    return 0;
                }
                return hg;
            }
        }

        int left = height(node->left), right = height(node->right);
        if (abs(left - right) > 1) isbalanced = false;
        return max(left, right) + 1; 
    }
};