Merge k-way - https://leetcode.com/problems/merge-k-sorted-lists/description/

gistfile1.txt

/**
 * 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) {
        throw new IllegalArgumentException();
      }
      
      final int k = lists.length;
      
      if (k == 1) {
        return lists[0];
      }
      
      // Let's do k-way merge
      PriorityQueue<ListNode> pq = new PriorityQueue<ListNode>(new Comparator<ListNode>() {
            @Override
            public int compare(ListNode o1, ListNode o2) {
                return o1.val - o2.val;
            }
        });
      
      // Add first k nodes from the list.
      for (ListNode node : lists) {
        if (node != null) {
          pq.offer(node);
        }
      }
      
      // Now iterate through the PQ, once popped, add next one from the list.
      ListNode tmpResult = new ListNode(-1);
      ListNode cur = tmpResult;
      while (pq.peek() != null) {
        ListNode nodeCur = pq.poll();
        cur.next = new ListNode(nodeCur.val);
        cur = cur.next;
        
        // check if more from the list
        ListNode nextCur = nodeCur.next;
        if (nextCur != null) {
          pq.offer(nextCur);
        }
      }
      
      return tmpResult.next;
    }
}

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