leetcode_Sum Root to Leaf Numbers _DFS_中等难度

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.

方法：看到hide tags知道是用DFS来做，所以往这方面想，方法并不难，比较简单，就是要注意递归边界、非法判断（空树等）。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int sum=0;
    int dfs(TreeNode *root,int tmp)
    {
        tmp=tmp*10+root->val;
        if(root->left==NULL && root->right==NULL)
        {
            sum+=tmp;
            return 1;
        }
        if(root->left!=NULL)
            dfs(root->left,tmp);
        if(root->right!=NULL)
            dfs(root->right,tmp);
    }
    
    int sumNumbers(TreeNode *root) {
        int tmp=0;
        if(root==NULL)
            return 0;
        dfs(root,tmp);
        return sum;
    }
};