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Merge K Sorted Lists

Certification: Advanced Level Accuracy: 0% Submissions: 0 Points: 15

Write a C program to merge k sorted linked lists and return it as one sorted list. You are given an array of k linked-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. 
    • Therefore, the result is [1,1,2,3,4,4,5,6]
    •
Example 2
    
  
  • Input: lists = []
    • 
  
  • Output: []
    • 
  
  • Explanation: 
    • No linked lists are provided. 
    • Return NULL (empty list). 
    • Therefore, the result is empty
    •
Constraints
    
  
  • k == lists.length
    • 
  
  • 0 ≤ k ≤ 10^4
    • 
  
  • 0 ≤ lists[i].length ≤ 500
    • 
  
  • -10^4 ≤ lists[i][j] ≤ 10^4
    • 
  
  • lists[i] is sorted in ascending order
    • 
  
  • The sum of lists[i].length will not exceed 10^4
    • 
  
  • Time Complexity: O(N log k) where N is total number of nodes
    • 
  
  • Space Complexity: O(1)
    •
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Hint
Solution

Solution Hints

    
  
  • Use divide and conquer approach to merge lists efficiently
    • 
  
  • Merge lists in pairs until only one list remains
    • 
  
  • Use a helper function to merge two sorted linked lists
    • 
  
  • Continue pairing and merging until all lists are combined
    • 
  
  • Handle edge cases like empty lists and single list
    •

Steps to solve by this approach:

 Step 1: Handle base cases - return NULL for empty array, return single list for array of size 1
 Step 2: Use divide and conquer approach by pairing adjacent lists
 Step 3: Merge each pair of lists using mergeTwoLists helper function
 Step 4: Update the lists array with merged results and reduce the size
 Step 5: Continue the process until only one merged list remains
 Step 6: In mergeTwoLists, use dummy node technique for easier list manipulation
 Step 7: Return the final merged list which contains all elements in sorted order

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