[leetcode]23. Merge k Sorted Lists归并k个有序链表

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

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

题意：

归并k个有序链表。

思路：

用一个最小堆minHeap，将所有链表的头结点放入

新建一个linkedlist存储结果

minHeap里面每remove一个node（最小堆保证了该元素为当前堆中最小）， 就加到新建linkedlist中，并将该node.next加入到堆里

代码：

 class Solution {
     public ListNode mergeKLists(ListNode[] lists) {
         // special case
         if(lists.length==0) return null;

         //1. priorityqueue ((o1,o2)-> o1.val, o2.val) acsending order
         PriorityQueue<ListNode> heap = new PriorityQueue<>((o1,o2)-> o1.val-o2.val);

         //add each list's each node into the heap, in acsending order
         int size = lists.length;
         for(int i = 0; i<size; i++){
             if(lists[i]!=null){
                 heap.add(lists[i]); //注意这步操作直接把list的每个节点都扔进了heap

             }
         }
         //2. create a new linkedlist
         ListNode fakeHead = new ListNode(-1);
         ListNode current = fakeHead;

         //3. add every node from priorityqueue's removing
         while(heap.size()!=0){
             ListNode node = heap.remove();
             current.next = node;
             current=current.next;
             if(node.next!=null) heap.add(node.next);
         }
       return fakeHead.next;
     }
 }