Sum Root to Leaf Numbers

/**
 * 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 sum;
    void dfs(TreeNode *root,int num)
    {
        if(root->left==NULL && root->right==NULL) sum+=num;
        if(root->left) dfs(root->left,num*10+root->left->val);
        if(root->right) dfs(root->right,num*10+root->right->val);
    }
    int sumNumbers(TreeNode *root) {
        if(root==NULL) return 0;
        sum=0;
        int num=0;
        dfs(root,root->val);
        return sum;
    }
};