Merge k Sorted Lists.java

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

/**
 * 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(ArrayList<ListNode> lists) {
    	if (lists == null || lists.isEmpty()) return null;

	Comparator<ListNode> comp = new Comparator<ListNode>() {
	    public int compare(ListNode o1, ListNode o2) {
		if (o1.val < o2.val) return -1;
		if (o1.val > o2.val) return 1;
		return 0;
	    }
	};
	PriorityQueue<ListNode> heap = new PriorityQueue<ListNode>(lists.size(), comp);
	for (ListNode node : lists) {
	    if (node != null) heap.add(node);
	}
	ListNode head = null, cur = null;
	while (!heap.isEmpty()) {
            if (head == null) {
	        head = heap.poll();
		cur = head;
	    } else {
		cur.next = heap.poll();
		cur = cur.next;
	    }
	    if (cur.next != null) heap.add(cur.next);
	}
	return head;
    }
}