Merge k Sorted Lists

Source

• lintcode: 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.

Java

/**
 * 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) {  
        ListNode dummy = new ListNode(0);
        ListNode tail = dummy;

        if (lists == null || lists.size() < 1) {
            return null;
        }
        PriorityQueue<ListNode> minHeap = new PriorityQueue<ListNode>(lists.size(), new Comparator<ListNode>() {
            public int compare(ListNode l1, ListNode l2) {
                return l1.val - l2.val;
            }    
        });
        for (int i = 0; i < lists.size(); i++) {
            if (lists.get(i) != null) {
                minHeap.offer(lists.get(i));
            }
        }
        while (!minHeap.isEmpty()) {
            ListNode temp = minHeap.poll();
            tail.next = temp;
            tail = temp;
            if (temp.next != null) {
                minHeap.offer(temp.next);
            }

        }
        return dummy.next;   
    }
}