def locate(key):
    ptr = head
    while ptr != null:
        if ptr.key == key:
            return ptr
        ptr = ptr.next
    return null

def del (ptr):
    if ptr == head: # head is special case
        head = ptr.next
        free ptr
        return

    ptr.prev.next = ptr.next
    free ptr

def del (ptr):
    if ptr == head: # head is special case
        head = ptr.next
        free ptr
        return

    prev = head
    while prev.next != ptr:
        prev = prev.next
    prev.next = ptr.next
    free ptr

def del (key):
    if head == null: # no action on empty list
        return

    if head.key == key: # head is special case
        temp = head
        head = head.next
        free temp
        return

    prev = head
    while prev.next != null:
        if prev.next.key == key:
            temp = prev.next
            prev.next = temp.next
            free temp
            return
        prev = prev.next