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.
1 class Solution { 2 public: 3 int sumNumbers(TreeNode *root) { 4 int sum = 0; 5 if(!root) return sum; 6 return preOrder(root,sum); 7 } 8 int preOrder(TreeNode *root, int sum) 9 { 10 if(!root) return 0; 11 sum = sum*10 + root->val; 12 if(root->left == NULL && root->right == NULL) 13 { 14 return sum; 15 } 16 return preOrder(root->left,sum)+preOrder(root->right,sum); 17 } 18 };