Merge k Sorted Lists

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

最简单的方法是使用PriorityQueue。PriorityQueue是个基于优先级堆的极大优先级队列。
此队列按照在构造时所指定的顺序对元素排序，既可以根据元素的自然顺序来指定排序，
也可以根据 Comparator 来指定，这取决于使用哪种构造方法。

public ListNode mergeKLists(ListNode[] lists) {if (lists.length == 0)return null;//PriorityQueue is a sorted queuePriorityQueue<ListNode> q = new PriorityQueue<ListNode>(lists.length,new Comparator<ListNode>() {public int compare(ListNode a, ListNode b) {return a.val-b.val;}});//add first node of each list to the queuefor (ListNode list : lists) {if (list != null)q.add(list);}ListNode head = new ListNode(0);ListNode p = head; // serve as a pointer/cursorwhile (q.size() > 0) {ListNode temp = q.poll();//poll() retrieves and removes the head of the queue - q. p.next = temp;//keep adding next element of each listif (temp.next != null)q.add(temp.next);p = p.next;}return head.next;}