queue.py

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


class Queue(object):

    head = None
    tail = None

    def enqueue(self, data):
        new_node = Node(data)
        if self.tail:
            self.tail.next = new_node
        self.tail = new_node
        if self.head is None:
            self.head = self.tail

    def dequeue(self):
        if self.head is None:
            return None
        data = self.head.data
        self.head = self.head.next
        if self.head is None:
            self.tail = None
        return data

    def peek(self):
        if self.head is None:
            return None
        return self.head.data

    def is_empty(self):
        return self.head is None


if __name__ == "__main__":
    q = Queue()
    q.enqueue(1)
    q.enqueue(2)
    q.enqueue(3)
    print(q.dequeue())
    print(q.dequeue())
    print(q.dequeue())
    q.enqueue(4)
    print(q.dequeue())
    q.enqueue(5)
    print(q.peek())
    print(q.dequeue())
    print(q.dequeue())
    print(q.peek())
    q.enqueue(1)
    q.enqueue(2)
    q.enqueue(3)
    q.enqueue(4)
    q.enqueue(5)
    print(q.peek())
    print(q.dequeue())
    print(q.dequeue())
    print(q.dequeue())
    print(q.peek())
    print(q.dequeue())
    print(q.dequeue())
    print(q.dequeue())