lintcode-medium-Merge k Sorted Lists

Merge k sorted linked lists and return it as one sorted list.

Analyze and describe its complexity.

Example

Given lists:

[
  2->4->null,
  null,
  -1->null
],

return -1->2->4->null.

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param lists: a list of ListNode
     * @return: The head of one sorted list.
     */
    public ListNode mergeKLists(List<ListNode> lists) {
        // write your code here

        if(lists == null || lists.size() == 0)
            return null;

        PriorityQueue<ListNode> queue = new PriorityQueue<ListNode>(lists.size(), new Comparator<ListNode>(){
                public int compare(ListNode p1, ListNode p2){
                    return p1.val - p2.val;
                }
            });
        ListNode fakehead = new ListNode(0);
        ListNode p = fakehead;

        for(int i = 0; i < lists.size(); i++){
            if(lists.get(i) != null)
                queue.offer(lists.get(i));
        }

        while(!queue.isEmpty()){
            ListNode temp = queue.poll();

            p.next = temp;
            p = p.next;

            if(temp.next != null)
                queue.offer(temp.next);
        }

        return fakehead.next;
    }
}