Merge k Sorted Lists

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

Solution:

  1. /**
  2.  * Definition for singly-linked list.
  3.  * public class ListNode {
  4.  *     int val;
  5.  *     ListNode next;
  6.  *     ListNode(int x) { val = x; }
  7.  * }
  8.  */
  9. public class Solution {
  10.   public ListNode mergeKLists(ListNode[] lists) {
  11.     ListNode f = new ListNode(0);
  12.     ListNode p = f;
  14.     int k = lists.length;
  16.     PriorityQueue<ListNode> heap = new PriorityQueue<>(k + 1, new Comparator<ListNode>() {
  17.       public int compare(ListNode a, ListNode b) {
  18.         return a.val - b.val;
  19.       }
  20.     });
  22.     // step 1. put the first node of every list to the min heap
  23.     for (int i = 0; i < k; i++) {
  24.       if (lists[i] != null) {
  25.         heap.offer(lists[i]);
  26.       }
  27.     }
  29.     while (!heap.isEmpty()) {
  30.       ListNode node = heap.poll();
  32.       p.next = node;
  33.       p = p.next;
  35.       if (node.next != null) {
  36.         heap.offer(node.next);
  37.       }
  38.     }
  40.     p.next = null;
  41.     return f.next;
  42.   }
  43. }