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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.

思路：

先序遍历的思想（根左右）+数字求和（每一层都是上层和*10+当前值）

1. class Solution  

2. {  

3. public:  

4.       

5.     int sumNumbers(TreeNode* root)  

6.     {  

7.         int sum = 0;  

8.         if (root==NULL)  

9.         {  

10.             return sum;  

11.         }  

12.   

13.         return preorderSumNum(root, sum);  

14.     }  

15.   

16.     int preorderSumNum(TreeNode* root,  int sum)  

17.     {  

18.         if (root==NULL)  

19.         {  

20.             return 0;  

21.         }  

22.   

23.         sum = sum * 10 + root->val;  

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

25.         {  

26.             return sum;  

27.         }  

28.   

29.         return preorderSumNum(root->left, sum) + preorderSumNum(root->right. sum);  

30.     }  

31. };