class Node:
    def __init__(self, value):
        self.value = value
        self.next = None


def insert_at_head(head, value):
    new_node = Node(value)
    new_node.next = head
    return new_node  # new_node is the new head


def insert_after(node, value):
    new_node = Node(value)
    new_node.next = node.next
    node.next = new_node


head = Node(20)
head = insert_at_head(head, 10)   # 10 -> 20
insert_after(head, 15)            # 10 -> 15 -> 20

current = head
while current:
    print(current.value, end=" ")
    current = current.next
# 10 15 20