【leetcode】【medium】129. Sum Root to Leaf Numbers

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.

Note: A leaf is a node with no children.

Example:

Input: [1,2,3]
    1
   / \
  2   3
Output: 25
Explanation:
The root-to-leaf path 1->2 represents the number 12The root-to-leaf path 1->3 represents the number 13Therefore, sum = 12 + 13 = 25

Example 2:

Input: [4,9,0,5,1]
    4
   / \
  9   0
 / \
5   1
Output: 1026
Explanation:
The root-to-leaf path 4->9->5 represents the number 495.
The root-to-leaf path 4->9->1 represents the number 491.
The root-to-leaf path 4->0 represents the number 40.
Therefore, sum = 495 + 491 + 40 = 1026

題目鏈接：https://leetcode-cn.com/problems/sum-root-to-leaf-numbers/

 

思路

法一：遞歸

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

總感覺還有更高級的遞歸寫法。。暫時還想不到。

法二：非遞歸

暫不練習，後面再補。