[LeetCode129]Sum Root to Leaf Numbers

题目：

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.

分类：Tree DFS 

代码：

/**
 * 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) {
        return DFS(root,0);
    }
    
    int DFS(TreeNode* root, int sum)
    {
        if(!root)
            return 0;
        int newSum = sum * 10 + root->val;
        if(!root->left && !root->right)
            return newSum;
        return DFS(root->left,newSum) + DFS(root->right,newSum);
    }
};