25_reverse_node_in_k_groups.go

// Source: https://leetcode.com/problems/reverse-nodes-in-k-group/
/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
 func reverseKGroup(head *ListNode, k int) *ListNode {
	dummy := &ListNode{Next: head}
	prev := dummy
	for head != nil {
		tail := prev
		for i := 0; i < k; i++ {
			tail = tail.Next
			if tail == nil {
				return dummy.Next
			}
		}
		next := tail.Next
		head, tail = reverse(head, tail)
		prev.Next = head
		tail.Next = next
		prev = tail
		head = tail.Next
	}
	return dummy.Next
 }

 func reverse(head, tail *ListNode) (*ListNode, *ListNode) {
	prev := tail.Next
	p := head
	for prev != tail {
		next := p.Next
		p.Next = prev
		prev = p
		p = next
	}
	return tail, head
}