【leetcode】Merge k Sorted Lists(按大小顺序连接k个链表)

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

题意：把k个排序成一个有序链表。

用优先队列先把k个链表遍历一遍把值存起来，在建一个新链表吧数从优先队列里一个个放进去，注意空指针的判断。

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode *mergeKLists(vector<ListNode *> &lists) {        int i,j,len=lists.size();        priority_queue<int,vector<int>,greater<int>> q;        int flag=1;        while(flag!=0)        {            flag=0;            for(i=0;i<len;++i)            {                if(lists[i])                {                    flag=1;                    q.push(lists[i]->val);                    lists[i]=lists[i]->next;                }            }        }        if(q.empty())return NULL;        ListNode *root,*now;        root=new ListNode(q.top());        root->next=NULL;        now=root;        q.pop();        while(!q.empty())        {             now->next=new ListNode(q.top());             now=now->next;             q.pop();             now->next=NULL;        }        return root;    }};