package main

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

/* func list2str(head *ListNode) string {
	curr := head
	str := make([]rune, 0)
	for curr != nil {
		str = append(str, rune(curr.Val+'0'))
		curr = curr.Next
	}
	return string(str)
} */

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func deleteDuplicates(head *ListNode) *ListNode {
	if head == nil || head.Next == nil {
		return head
	}
	dummy := ListNode{0, head}
	slow := &dummy
	fast := slow.Next
	for fast != nil {
		if fast.Next == nil || fast.Next.Val != slow.Next.Val {
			// duplicates end
			if slow.Next != fast {
				// cut dup
				slow.Next = fast.Next
				fast = fast.Next
				// do not move ptr in case of fast maybe nil
				continue
			}
			// no duplicates, move both slow & fasr
			slow = slow.Next
		}
		// if has dup, move fast only
		fast = fast.Next
	}
	return dummy.Next
}

/* func main() {
	l15 := ListNode{5, nil}
	l14 := ListNode{5, &l15}
	l13 := ListNode{4, &l14}
	l12 := ListNode{3, &l13}
	l1 := &ListNode{1, &l12}
	fmt.Println(list2str(deleteDuplicates(l1)))
	fmt.Println(list2str(deleteDuplicates(nil)))
} */