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Merge K Sorted Lists

Certification: Advanced Level Accuracy: 100% Submissions: 2 Points: 15

Write a Java program to merge k sorted linked lists into one sorted linked list. You need to use a priority queue (min heap) to efficiently merge these lists.

Example 1
    
  
  • Input: lists = [[1,4,5],[1,3,4],[2,6]]
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  • Output: [1,1,2,3,4,4,5,6]
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  • Explanation: We have 3 sorted linked lists: [1->4->5], [1->3->4], and [2->6]. When merged, the resulting linked list is [1->1->2->3->4->4->5->6].
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Example 2
    
  
  • Input: lists = []
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  • Output: []
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  • Explanation: The input is an empty array, so there are no lists to merge.
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Constraints
    
  
  • k == lists.length
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  • 0 <= k <= 10^4
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  • 0 <= lists[i].length <= 500
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  • -10^4 <= lists[i][j] <= 10^4
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  • lists[i] is sorted in ascending order
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  • The sum of lists[i].length will not exceed 10^4
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  • Time Complexity: O(N log k) where N is the total number of nodes and k is the number of linked lists
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  • Space Complexity: O(k)
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Priority QueueShopifyD. E. Shaw
Editorial

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Hint
Solution

Solution Hints

    
  
  • Use a priority queue (min heap) to always get the smallest node among the k lists
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  • Initialize the priority queue with the first node of each list
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  • Keep polling the smallest node from the priority queue and adding it to the result list
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  • After polling a node, add its next node to the priority queue if it exists
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  • Continue until the priority queue is empty
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  • Handle edge cases like empty lists or lists with null values
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Steps to solve by this approach:

 Step 1: Handle edge cases - if the input array is null or empty, return null.

 Step 2: Create a min heap (priority queue) that orders nodes based on their values.

 Step 3: Add the first node of each non-null list to the min heap.

 Step 4: Create a dummy node to start building the result linked list.

 Step 5: While the min heap is not empty, poll the smallest node, append it to the result list.

 Step 6: After appending a node to the result, add its next node to the min heap if it exists.

 Step 7: Return the next of the dummy node as the head of the merged list.

Submitted Code :