LeetCode - Hard - 23. Merge k Sorted Lists

Topic

• Linked List
• Divide and Conquer
• Heap

Description

https://leetcode.com/problems/merge-k-sorted-lists/

You are given an array of k linked-lists lists, each linked-list is sorted in ascending order.

Merge all the linked-lists into one sorted linked-list and return it.

Example 1:

Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
Explanation: The linked-lists are:
[
  1->4->5,
  1->3->4,
  2->6
]
merging them into one sorted list:
1->1->2->3->4->4->5->6

Example 2:

Input: lists = []
Output: []

Example 3:

Input: lists = [[]]
Output: []

Constraints:

• k == lists.length
• 0 <= k <= 10^4
• 0 <= lists[i].length <= 500
• -10^4 <= lists[i][j] <= 10^4
• lists[i] is sorted in ascending order.
• The sum of lists[i].length won’t exceed 10^4.

Analysis

LeetCode 21. Merge Two Sorted Lists的延伸。

Submission

import com.lun.util.SinglyLinkedList.ListNode;

public class MergeKSortedLists {

	public ListNode mergeKLists(ListNode[] lists) {
		ListNode result = null;
		if (lists == null)
			return result;

		for (ListNode list : lists) {
			result = mergeTwoList(result, list);
		}

		return result;
	}

	private ListNode mergeTwoList(ListNode l1, ListNode l2) {
		if (l1 == null) {
			return l2;
		} else if (l2 == null) {
			return l1;
		} else if (l1.val < l2.val) {
			l1.next = mergeTwoList(l1.next, l2);
			return l1;
		} else {
			l2.next = mergeTwoList(l1, l2.next);
			return l2;
		}
	}

}

Test

import static org.junit.Assert.*;
import static com.lun.util.SinglyLinkedList.*;

import org.junit.Test;

import com.lun.util.SinglyLinkedList.ListNode;

public class MergeKSortedListsTest {

	@Test
	public void test() {
		MergeKSortedLists obj = new MergeKSortedLists();

		ListNode[] lists1 = {ints2List(1, 4, 5), ints2List(1, 3, 4), ints2List(2, 6)};
		ListNode expected1 = ints2List(1, 1, 2, 3, 4, 4, 5, 6);
		
		assertTrue(areTwoListEqual(obj.mergeKLists(lists1), expected1));
		
		ListNode[] list2 = null;
		assertNull(obj.mergeKLists(list2));
		
		ListNode[] list3 = {};
		assertNull(obj.mergeKLists(list3));
	}
}