[leetcode]Sum Root to Leaf Numbers

对这种树的操作，已经比较熟练了。

class Solution {

public:

    int sumNumbers(TreeNode *root) {

        int sum = 0;

        calculate(root, 0, sum);

        return sum;

    }

    

    void calculate(TreeNode *root, int currentSum, int& sum)

    {

        if (root == NULL) return;

        currentSum = currentSum * 10 + root->val;

        if (root->left == NULL && root->right == NULL)

        {

            sum += (currentSum);

        }

        else

        {

            calculate(root->left, currentSum, sum);

            calculate(root->right, currentSum, sum);

        }

    }

};