25. Reverse Nodes in k-Group

Problem

Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.

Example:

Given this linked list: 1->2->3->4->5

For k = 2, you should return: 2->1->4->3->5

For k = 3, you should return: 3->2->1->4->5

Note:

• Only constant extra memory is allowed.
• You may not alter the values in the list's nodes, only nodes itself may be changed.

Problem Summary

Reverse the linked list in groups of every K elements. If there are not K elements, do not reverse them.

Solution Approach

This problem is an enhanced version of problem 24. Problem 24 reverses the linked list by swapping every two adjacent elements. Problem 25 requires reversing every k adjacent elements in the linked list; problem 24 is equivalent to the special case where k = 2.

Code

package leetcode

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func reverseKGroup(head *ListNode, k int) *ListNode {
	node := head
	for i := 0; i < k; i++ {
		if node == nil {
			return head
		}
		node = node.Next
	}
	newHead := reverse(head, node)
	head.Next = reverseKGroup(node, k)
	return newHead
}

func reverse(first *ListNode, last *ListNode) *ListNode {
	prev := last
	for first != last {
		tmp := first.Next
		first.Next = prev
		prev = first
		first = tmp
	}
	return prev
}