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Reverse Nodes in k-Group

Certification: Advanced Level Accuracy: 0% Submissions: 0 Points: 10

Write a C# program to reverse the nodes of a linked list k at a time, where k is a positive integer. If the number of nodes is not a multiple of k, then the remaining nodes should remain as is. You may not alter the values in the list's nodes, only nodes themselves may be changed.

Example 1
    
  
  • Input: head = [1,2,3,4,5], k = 2
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  • Output: [2,1,4,3,5]
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  • Explanation:
    
      
      
    • First group: [1,2] -> reverse -> [2,1]
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    • Second group: [3,4] -> reverse -> [4,3]
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    • Third group: [5] (less than k nodes, remain as is) -> [5]
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    • Result: [2,1,4,3,5]
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Example 2
    
  
  • Input: head = [1,2,3,4,5], k = 3
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  • Output: [3,2,1,4,5]
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  • Explanation:
    
      
      
    • First group: [1,2,3] -> reverse -> [3,2,1]
      • 
      
    • Second group: [4,5] (less than k nodes, remain as is) -> [4,5]
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    • Result: [3,2,1,4,5]
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Constraints
    
  
  • The number of nodes in the list is n
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  • 1 <= k <= n <= 5000
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  • 0 <= Node.val <= 1000
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  • Time Complexity: O(n)
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  • Space Complexity: O(1) extra space (excluding recursion stack)
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Hint
Solution

Solution Hints

    
  
  • Count k nodes to determine if there are enough to reverse
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  • Use the standard three-pointer technique to reverse a linked list segment
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  • Keep track of connections before and after each group
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  • For each group, reverse k nodes and update connections
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  • Handle the case when there are fewer than k nodes at the end
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Steps to solve by this approach:

 Step 1: Create a dummy node to handle edge cases and simplify the code.
 Step 2: Count the total number of nodes in the list.
 Step 3: Use three pointers to reverse each group of k nodes: prev (points to the node before current group), curr (points to the first node of current group), and next.
 Step 4: For each group, reverse k nodes using the standard three-pointer technique.
 Step 5: After reversing a group, connect the reversed segment back to the main list.
 Step 6: Update pointers to prepare for the next group.
 Step 7: If there are fewer than k nodes left, leave them as is.

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