LeetCode 129 Sum Root to Leaf Numbers

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int ans =0;
    int sumNumbers(TreeNode* root) {
        ans =0;
       dfs(root,0);
        return ans;
        
    }
    
    void dfs(TreeNode* root,int num)
    {
        if(root==NULL) return;
        if(root->left!=NULL)
        {
            dfs(root->left,num*10+root->val);
        }
        if(root->right!=NULL)
        {
            dfs(root->right,num*10+root->val);
        }
        if(root->left==NULL&&root->right==NULL)
            ans+=num*10+root->val;
    }
        
};