https://leetcode.com/problems/remove-duplicates-from-sorted-list/description/
func deleteDuplicates(head *ListNode) *ListNode {
if head == nil {
return nil
}
mark := head
p := head.Next
for p != nil {
if p.Val == mark.Val {
mark.Next = p.Next
} else {
mark = p
}
p = p.Next
}
return head
}