【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;

    }

};