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Reverse Nodes in K-Sized Groups

ByteDanceSoftware Engineer · Technical Screen
MediumSoftware EngineerTechnical ScreenCoding & Algorithms

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This problem tests your ability to manipulate linked list pointers by reversing sub-segments of a list while maintaining structural integrity.

Problem Statement

Given the head of a singly linked list and an integer k, reverse the nodes of the list k at a time, and return the modified list.

If the number of nodes is not a multiple of k, the left-out nodes at the end should remain as they are. You may not alter the values in the list's nodes; only the nodes themselves may be changed.

Function Signature

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def reverseKGroup(head: ListNode, k: int) -> ListNode:
    pass

Constraints

  • The number of nodes in the list is n.
  • 1 <= k <= n <= 5000
  • 0 <= Node.val <= 1000

Examples

Example 1: Input: head = [1, 2, 3, 4, 5], k = 2 Output: [2, 1, 4, 3, 5] Explanation: The first two nodes (1, 2) are reversed, the next two (3, 4) are reversed, and the last node (5) stays as is.

Example 2: Input: head = [1, 2, 3, 4, 5], k = 3 Output: [3, 2, 1, 4, 5] Explanation: The first three nodes (1, 2, 3) are reversed, leaving (4, 5) untouched.

Edge Cases

  • k = 1: The list should remain unchanged.
  • n < k: The entire list should remain unchanged.
  • n is a multiple of k.

Hints

Hint 1

Determine if there are at least k nodes remaining in the list before attempting to reverse a group.

Hint 2

Use a helper function to reverse a segment of the linked list of length k, keeping track of the previous and next pointers to stitch the segments back together.

Hint 3

Use a dummy head node to simplify the logic when the head of the list itself needs to be swapped.

Follow-up questions

  • How would you modify your approach to solve this problem if the linked list was doubly linked?
  • Can you implement this using a recursive approach, and what would the space complexity be compared to an iterative solution?
  • What changes would be required if the groups were to be reversed only if they contain exactly k nodes (i.e., discard the remaining nodes if they are fewer than k)?