題目

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.

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 12.
The root-to-leaf path 1->3 represents the number 13.
Therefore, 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.

解答

今天又選了一道水題，就是二叉樹的先序遍歷，又是一遍AC。。。感覺這種題目都不用動腦子，明天選一道要動腦子的題目。

下面是AC的代碼：

/**
 * 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 sum = 0
 
;
    int temp = 0;
    void preorder(TreeNode *node){
        if(node == NULL){
            return ;
        }
        temp = temp * 10 + node->val;
        if(node->left == NULL && node->right == NULL){
            sum += temp;
        }
        else{
            preorder(node->left);
            preorder(node->right);
        }
        temp = (temp - node->val) / 10;
        return ;
    }
    int sumNumbers(TreeNode* root) {
        preorder(root);
        return sum;
    }
};

138

LeetCode OJ 129. Sum Root to Leaf Numbers