[leetCode]Merge k Sorted Lists

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

外部排序的基础

#include <iostream>
#include<vector>
using namespace std;

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};
class Solution {
public:
    static const int MAX = 9999;
    ListNode *mergeKLists(vector<ListNode *> &lists) {
        int listnum = checkListNum(lists);
        ListNode *head,*tail;
        head = tail = NULL;
        while(listnum){
            int index,min;
            min = MAX;
            index = 0;
            for(int i = 0; i < lists.size();i++){
                if(lists[i] != NULL){
                    if(lists[i]->val < min){
                        min = lists[i]->val;
                        index = i;
                    }
                }
            }
            if(head == NULL){
                head = lists[index];
                tail = head;
            }else{
                tail->next = lists[index];
                tail = tail->next;
            }
            lists[index] = lists[index]->next;
            tail->next = NULL;
            listnum = checkListNum(lists);
        }
        return head;
    }
    int checkListNum(vector<ListNode *> &lists){
        int num = 0;
        for(int i = 0; i < lists.size();i++){
            if(lists[i] != NULL) num++;
        }
        return num;
    }
};