LeetCode刷题笔录Merge k Sorted Lists

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

用一个priority queue，先把每个list的第一个node放进去。每次拿一个node出来，并把node.next放进去。事实上排序是由heap来做的，感觉像是作弊的样子。时间复杂度O(nklogk)。n是元素总数。

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { *         val = x; *         next = null; *     } * } */public class Solution {    public ListNode mergeKLists(List<ListNode> lists) {    Comparator<ListNode> comparator = new Comparator<ListNode>() {@Overridepublic int compare(ListNode o1, ListNode o2) {return o1.val - o2.val;}};        PriorityQueue<ListNode> queue = new PriorityQueue<ListNode>(50, comparator);        for(ListNode node : lists){        if(node != null)        queue.offer(node);        }                ListNode head = null;        ListNode pre = null;        while(!queue.isEmpty()){        ListNode node = queue.poll();        if(head == null){        head = node;        pre = node;        }        else{        pre.next = node;        }                pre = node;        if(node.next != null){        queue.offer(node.next);        }        }                return head;    }}