Interview Question Merge k Sorted Lists

By Fatih KABAKCI

The problem is to merge k-sorted lists and return it as a new one. We can take an inspiration the problem Merge Two Sorted Lists. There was only two lists in that problem. Now, we have been facing to merge multiple lists. However, we can still use the same method that we defined in there.

Problem #23 - Merge k Sorted Lists

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

Difficulty: Hard

Solution in Java

package com.fatihkabakci.Hard.MergekSortedLists;

import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;

/**
 * @author fkabakci
 * 
 * Problem Description: Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.
 * 
 * Solution:
 * 
 * The probiem is about merging k sorted lists. Take an inspiration the problem based on 2 sorted lists.
 * 
 * 0. Use the method mergeTwoLists() which was coded in 2-sorted lists problem.
 * 1. Move all elements of the array into a list.
 * 2. Get two nodes of front and merge them. Then add them end of the list until the list has only one node.
 */

public class MergekSortedLists {
	public ListNode mergeKLists(ListNode[] lists) {
		List<ListNode> l = Arrays.asList(lists);
		LinkedList<ListNode> o = new LinkedList<ListNode>(l);
		while (o.size() > 1) {
			ListNode head = o.poll();
			ListNode tail = o.poll();
			o.add(mergeTwoLists(head, tail));
		}
		return o.poll();
	}

	public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
		ListNode head = new ListNode(0);
		ListNode l3 = head;
		while (l1 != null || l2 != null) {
			int val1 = (l1 != null) ? l1.val : Integer.MAX_VALUE;
			int val2 = (l2 != null) ? l2.val : Integer.MAX_VALUE;
			l3.next = new ListNode(Math.min(val1, val2));
			l3 = l3.next;
			if (val1 < val2)
				l1 = l1.next;
			else
				l2 = l2.next;
		}
		return head.next;
	}

	public static void main(String[] args) {
		MergekSortedLists r = new MergekSortedLists();
		ListNode head = Data.init(new int[] { 1, 2, 3 });
		ListNode tail = Data.init(new int[] { 4, 5, 6 });
		ListNode[] list = { head, tail };
		ListNode entire = r.mergeKLists(list);
		while (entire != null) {
			System.out.print(entire.val + " ->");
			entire = entire.next;
		}
	}
}

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Merge Two Sorted Lists