题目要求：
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.
这道题利用深度优先搜索即可，具体程序（4ms）如下：
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
int sumNumbers(TreeNode* root) {
int totalSum = ;
sumNumbersSub(root, , totalSum);
return totalSum;
} void sumNumbersSub(TreeNode *tree, int sum, int &totalSum)
{
if(!tree)
return; sum = sum * 10 + tree->val;
if(!tree->left && !tree->right)
{
totalSum += sum;
return;
} sumNumbersSub(tree->left, sum, totalSum);
sumNumbersSub(tree->right, sum, totalSum);
} };