Sum Root to Leaf Numbers

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

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

For example,

    1   / \  2   3

The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

这个题挺简单，但是我还是出了一点问题，一开始我以为第归终止简单的就是root == NULL， 提交了发现这样会重复计数，因为每个叶结点有两个空子节点。大意了。

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