LeetCode 023 Merge k Sorted Lists

题目要求：Merge k Sorted Lists

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

Analysis

The simplest solution is using PriorityQueue. The elements of the priority queue are ordered according to their natural ordering, or by a comparator provided at the construction time (in this case).

Java:

import java.util.ArrayList;
import java.util.Comparator;
import java.util.PriorityQueue;
 
//  Definition for singly-linked list.
class ListNode {
    int val;
    ListNode next;
 
    ListNode(int x) {
        val = x;
        next = null;
    }
}
 
public class Solution {
    public ListNode mergeKLists(ArrayList<ListNode> lists) {
        if (lists.size() == 0)
            return null;
 
        //PriorityQueue is a sorted queue
        PriorityQueue<ListNode> q = new PriorityQueue<ListNode>(lists.size(),
                new Comparator<ListNode>() {
                    public int compare(ListNode a, ListNode b) {
                        if (a.val > b.val)
                            return 1;
                        else if(a.val == b.val)
                            return 0;
                        else 
                            return -1;
                    }
                });
 
        //add first node of each list to the queue
        for (ListNode list : lists) {
            if (list != null)
                q.add(list);
        }
 
        ListNode head = new ListNode(0);
        ListNode p = head; // serve as a pointer/cursor
 
        while (q.size() > 0) {
            ListNode temp = q.poll();
            //poll() retrieves and removes the head of the queue - q. 
            p.next = temp;
 
            //keep adding next element of each list
            if (temp.next != null)
                q.add(temp.next);
 
            p = p.next;
        }
 
        return head.next;
    }
}

Time: log(k) * n.
k is number of list and n is number of total elements.