LeetCode129. 求根到叶子节点数字之和

class Solution {
public:

    int sum = 0;

    int sumNumbers(TreeNode* root) {
      

        if (root == NULL)
        {
            return 0;
        }

        string tmpSum = "";


        myDfs(root, tmpSum);

        return sum;
    }

    int str2int(string s)
    {
        stringstream ss;
        ss << s;

        int i;
        ss >> i;
        return i;

    }
    void myDfs(TreeNode* root, string tmpSum)
    {
        if (NULL == root)
        {
            return;
        }

        tmpSum += to_string(root->val);

        if (root->left == NULL && root->right == NULL)
        {
            sum += str2int(tmpSum);
            return;
        }
        
        myDfs(root->left, tmpSum);
        myDfs(root->right, tmpSum);
    }
};