Is Merge k Sorted Lists easy or hard?

Merge k Sorted Lists is classified as a hard problem because it combines linked list manipulation with advanced algorithm design. Efficient solutions require understanding heaps or divide and conquer techniques to achieve O(N log k) performance.

Merge k Sorted Lists Python/Java solution

Python typically uses the heapq module to implement the min-heap, while Java uses PriorityQueue. Both implementations push the head nodes of each list, repeatedly extract the smallest value, and append it to the result list.

How to solve Merge k Sorted Lists in O(N log k)?

Use either a priority queue or a divide and conquer strategy. The heap approach always extracts the smallest node among the current list heads in O(log k) time. Since every node is processed once, the total runtime becomes O(N log k).

What is the best approach for Merge k Sorted Lists?

The most practical approach uses a min-heap (priority queue). Insert the head node of each of the k lists into the heap, repeatedly extract the smallest node, and push its next node. This keeps the heap size at most k and processes all N nodes in O(N log k) time with O(k) extra space.

Is Merge k Sorted Lists asked at Google/Amazon/Meta?

Merge k Sorted Lists appears frequently in interviews at companies like Google, Amazon, Meta, and Microsoft. It tests understanding of linked lists, heaps, and efficient multi-way merging techniques.

What data structure is used in Merge k Sorted Lists?

The most common data structure used is a min-heap (priority queue) to track the smallest node among the k lists. Linked list pointers are used to build the final merged list as nodes are extracted from the heap.

What is the time complexity of Merge k Sorted Lists?

The optimal solutions run in O(N log k) time where N is the total number of nodes across all lists and k is the number of lists. Both the min-heap approach and the divide and conquer merging strategy achieve this complexity.

Description Solution Video 10:34

23. Merge k Sorted Lists

Hard60.3% acceptance50 companies

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: