an interseting binary tree questions

题目：

Given a binary tree containing digits from0-9only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path1->2->3which represents the number123.

Find the total sum of all root-to-leaf numbers.

For example,

1

/

2 3

 

The root-to-leaf path1->2represents the number12.

The root-to-leaf path1->3represents the number13.

Return the sum = 12 + 13 =25.

 

class Solution {
public:
    int sumNumbers(TreeNode *root) {
        int sum = 0;
        if(!root) return sum;
        return preOrder(root,sum);
    }
    int preOrder(TreeNode *root, int sum)
    {
        if(!root) return 0;
         sum = sum*10 + root->val;
        if(root->left == NULL && root->right == NULL)
        {
            return sum;
        }        
        return preOrder(root->left,sum)+preOrder(root->right,sum);
    }
};