Balanced Binary Tree

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int flag=true;
    int dfs(TreeNode *root)
    {
         if(root==NULL) return true;
         int h1,h2;
         if(root->left==NULL) h1=0;
         else h1=dfs(root->left);
         if(root->right==NULL) h2=0;
         else h2=dfs(root->right);
         if(abs(h1-h2)>1) flag=0;
         return max(h1,h2)+1;
    }
    bool isBalanced(TreeNode *root) {
        dfs(root);
        return flag;
    }
};