129 Sum Root to Leaf Numbers

链接：https://leetcode.com/problems/sum-root-to-leaf-numbers/
问题描述：
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.

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