LeetCode 23. Merge k Sorted Lists

23. Merge k Sorted Lists

Difficulty:: Hard

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

Example:

Input:
[
  1->4->5,
  1->3->4,
  2->6
]
Output: 1->1->2->3->4->4->5->6

Solution

Language: Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists == null || lists.length == 0) {
            return null;
        }
        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;
        PriorityQueue<ListNode> pq = new PriorityQueue<>((a, b) -> a.val - b.val);
        for (ListNode l : lists) {
            if (l == null) {
                continue;
            }
            pq.offer(l);
        }
        while (!pq.isEmpty()) {
            ListNode tmp = pq.poll();
            cur.next = tmp;
            cur = cur.next;
            if (tmp.next != null) {
                pq.offer(tmp.next);
            }
        }
        return dummy.next;
    }
}

或者可以用归并排序的方法