LeetCode | # 23

/**
 * 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) {
        int k = lists.size();
        if(k==0)return null;
		PriorityQueue<ListNode> heap = new PriorityQueue<ListNode>(
				k, new Comparator<ListNode>(){
					@Override
					public int compare(ListNode o1, ListNode o2) {
						return o1.val-o2.val;
					}
				});
		for(int i=0; i<k; i++){
			ListNode node = lists.get(i);
			if(node != null){
				heap.offer(node);
			}
		}
		ListNode head = new ListNode(0), pre = head;
		while(heap.size()>0){
			ListNode c = heap.poll();
			pre.next = c;
			pre = pre.next;
			if(c.next != null)
				heap.offer(c.next);
		}
		return head.next;
    }
}