leetcode 23. Merge k Sorted Lists 优先级队列

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

Example:

Input:
[
1->4->5,
1->3->4,
2->6
]
Output: 1->1->2->3->4->4->5->6

思路：用一个 prority_queue 来存放所有的链表，每次取顶端元素即可。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    struct compare{
        bool operator()(const ListNode* a,const ListNode* b){
            return a->val>b->val;
        }
    };
    ListNode *mergeKLists(vector<ListNode*> &lists){
        priority_queue<ListNode*, vector<ListNode*>, compare> q;
        int sz=lists.size();
        if(!sz)return NULL;
        for(int i=0;i<sz;i++)
            if(lists[i])q.push(lists[i]);
        if(q.empty())return NULL;
        ListNode* res=q.top();q.pop();
        if(res->next)q.push(res->next);
        ListNode* p=res;
        while(!q.empty()){
            p->next=q.top();q.pop();
            p=p->next;
            if(p->next)q.push(p->next);
        }
        return res;
    }
};