Leetcode 23. Merge k Sorted Lists

原题地址：https://leetcode.com/problems/merge-k-sorted-lists/description/

题目描述

23.Merge k Sorted Lists
Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

意思很好懂，把k个有序链表合并成一个

思路

我的思路是就把这个当成归并排序里归并的那部分来实现

代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* merge(vector<ListNode*>& lists, int index1,int index2){
        if(lists[index1]==NULL && lists[index2]==NULL){
            return NULL;
        }
        if(lists[index1]==NULL){
            return lists[index2];
        }
        if(lists[index2]==NULL){
            return lists[index1];
        }
        ListNode * temp1 = lists[index1];
        ListNode * temp2 = lists[index2];
        ListNode * head;
        if(temp1->val > temp2->val){
            head = temp2;
            temp2=temp2->next;
        }else{
            head = temp1;
            temp1= temp1->next;
        }
        ListNode * current = head;
        while(temp1!=NULL && temp2 !=NULL){
            if(temp1->val > temp2-> val){
                current->next = temp2;
                current = current->next;
                temp2 = temp2->next;
            }else{
                current->next = temp1;
                current = current->next;
                temp1 = temp1->next;
            }
        }
        if(temp1!=NULL){
            current->next = temp1;
        }else{//temp2！=NULL
            current->next = temp2;
        }
        return head;   
    }
    
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        if(lists.empty()){
            return NULL;
        }
        if(lists.size()==1){
            return lists[0];
        }
        vector<ListNode*> new_lists;
        for(int i =0;i<lists.size();){
            ListNode * temp;
            if(i+1<lists.size()){
                temp = merge(lists,i,i+1);
            }else{
                temp = lists[i];
            }
            new_lists.push_back(temp);
            i+=2;
        }
        return mergeKLists(new_lists);
    }
};

踩坑

提交过程中有几次TLE和Runtime Error,TLE是因为merge函数里指针没处理好（赋值给head之后没有把temp移动到下一个元素，不过具体的情况我还是没想象清楚，先留着），RuntimeError是因为测试的例子里有的list是空的或者整个vector是空的没考虑到。