package main

import "fmt"

//type ListNode struct {
//	Val  int
//	Next *ListNode
//}

func reverseList(head *ListNode) *ListNode {
	pre := head
	next := head.Next
	head.Next = nil

	for next != nil {
		tmp := next.Next
		next.Next = pre
		pre = next
		next = tmp
	}

	return pre
}

func main() {
	root := &ListNode{
		Val: 1,
		Next: &ListNode{
			Val: 2,
			Next: &ListNode{
				Val: 3,
				Next: &ListNode{
					Val:  4,
					Next: nil,
				},
			},
		},
	}
	head := reverseList(root)
	for head != nil {
		fmt.Println(head.Val)
		head = head.Next
	}
}