MergekSortedLists.py

"""
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: lists = []
Output: []
Example 3:

Input: lists = [[]]
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.
"""

import heapq


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


class Solution:
    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
        min_heap = []
        dummy_head = ListNode(0)
        current = dummy_head

        # Add the head of each linked list to the min heap
        for i, head in enumerate(lists):
            if head:
                heapq.heappush(min_heap, (head.val, i, head))

        while min_heap:
            val, i, node = heapq.heappop(min_heap)
            current.next = node
            current = current.next
            if node.next:
                heapq.heappush(min_heap, (node.next.val, i, node.next))

        return dummy_head.next


# Create linked lists
lists = [
    ListNode(1, ListNode(4, ListNode(5))),
    ListNode(1, ListNode(3, ListNode(4))),
    ListNode(2, ListNode(6))
]

solution = Solution()
merged_list = solution.mergeKLists(lists)