master-algorithms-py/src/examples_book/adt/queues/linked_queue.py

#!/usr/bin/python3
# mari von steinkirch @2013
# steinkirch at gmail

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

class LinkedQueue(object):
    ''' Queue acts as a container for nodes (objects) that are inserted and removed according FIFO'''
    def __init__(self):
        self.front = None
        self.back = None

    def isEmpty(self):
        return bool(self.front)

    def dequeue(self):
        if self.front:
            value = self.front.value
            self.front = self.front.next
            return value
        raise Exception('Queue is empty, cannot dequeue.')

    def enqueue(self, value):
        node = Node(value)
        if self.front:
            self.back.next = node
            self.back = node
        else:
            self.front = node
            self.back = node
        return True

    def size(self):
        node = self.front
        num_nodes = 1
        while node.next:
            num_nodes += 1
            node = node.next
        return num_nodes

    def peek(self):
        return self.front.value


def main():
    queue = LinkedQueue()
    queue.enqueue(1)
    queue.enqueue(2)
    queue.enqueue(3)
    print(queue.size())    
    print(queue.peek())   
    print(queue.dequeue())  
    print(queue.peek())    


if __name__ == '__main__':
    main()