Merge K Sorted Lists

This page explains Java solution to problem Merge K Sorted Lists using Priority Queue data structure.

Problem Statement

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.

Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]

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

Solution

package com.vc.hard;

import java.util.*;

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */

class MergeKSortedLists {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists == null || lists.length == 0) return null;

        PriorityQueue<ListNode> pq = new PriorityQueue<>(new Comparator<ListNode>(){
            public int compare(ListNode l1, ListNode l2) {
                return Integer.compare(l1.val, l2.val);
            }
        });

        for(ListNode list: lists) {
            if(list != null)
                pq.offer(list);
        }

        ListNode dummy = new ListNode(-1);
        ListNode head = dummy;
        while(!pq.isEmpty()) {
            ListNode node = pq.poll();
            dummy.next = node;
            dummy = dummy.next;
            if(node.next != null)
                pq.offer(node.next);
        }
        return head.next;
    }
}

Time Complexity

O(N * log(K))
Where N is total number of elements in the final list and K is number of input lists.

Space Complexity

O(K) Where K is number of input lists.