【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;
    }
};