leetcode-129. Sum Root to Leaf Numbers

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.

思路：递归，每个节点保存临时和结果，遇到叶子节点就加入总和。

Acceped Code：

/**
 * 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 {
private:
    int sum;
public:
    int sumNumbers(TreeNode* root) {
        sum=0;
        if(root==nullptr)
        return 0;
        dfs(root,0);
        return sum;
    }
    
    void dfs(TreeNode* root,int presum)
    {
        if(root==nullptr)
        return;
        if(root->left==nullptr&&root->right==nullptr)
        {
            sum+=(presum*10+root->val);
        }else
        {
            int temp=presum*10+root->val;
            dfs(root->left,temp);
            dfs(root->right,temp);
        }
    }
};