README.md

23. Merge k Sorted Lists

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.

Example 1:

Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
Explanation: The linked-lists are:
[
  1->4->5,
  1->3->4,
  2->6
]
merging them into one sorted list:
1->1->2->3->4->4->5->6

Example 2:

Input: list = []
Output: []

Example 3:

Input: [[]]
Output: []

Constraints:

• k == lists.length
• 0 <= k <= 104
• 0 <= lists[i].length <= 500
• -104 <= lists[i][j] <= 104
• lists[i] is sorted in ascending order.
• The sum of lists[i].length will not exceed 104.

Solutions (Python)

1. Solution

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

import heapq


class Solution:
    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
        heap = [(lists[i].val, i)
                for i in range(len(lists)) if lists[i] is not None]
        heapq.heapify(heap)
        hair = ListNode()
        curr = hair

        while len(heap) > 0:
            _, i = heapq.heappop(heap)
            curr.next = lists[i]
            curr = curr.next
            lists[i] = lists[i].next
            if lists[i] is not None:
                heapq.heappush(heap, (lists[i].val, i))

        return hair.next