Implementing a Queue with Basic Operations

You are required to implement a queue data structure that supports the following operations:

• enqueue x: Add an element x to the end of the queue.
• dequeue: Remove and return the front element of the queue. If the queue is empty, return None.
• front: Return the front element of the queue without removing it. If the queue is empty, return None.
• size: Return the number of elements in the queue.
• empty: Return True if the queue is empty, otherwise return False.

The operations are given as commands. The enqueue command is followed by an integer x. For operations that return a value (dequeue, front, size, and empty), output the corresponding result on a separate line.

Note: In terms of algorithm, the queue follows the FIFO (first-in-first-out) principle. Mathematically, if we denote the state of the queue as a sequence \(Q = [q_1, q_2, \ldots, q_n]\), then the operations can be described as follows:

• \(\text{enqueue}(x): Q \gets [q_1, q_2, \ldots, q_n, x]\)
• \(\text{dequeue}(): \text{if } n > 0, \text{ return } q_1 \text{ and update } Q \gets [q_2, \ldots, q_n], \text{ otherwise return } \texttt{None}\)
• \(\text{front}(): \text{if } n > 0, \text{ return } q_1, \text{ otherwise return } \texttt{None}\)
• \(\text{size}(): \text{ return } n\)
• \(\text{empty}(): \text{ return } \texttt{True} \text{ if } n=0 \text{, otherwise } \texttt{False}\)

inputFormat

The first line of input contains a single integer n (1 ≤ n ≤ 10⁴), representing the number of commands. The following n lines each contain one command. The commands can be one of the following formats:

• enqueue x - where x is an integer.
• dequeue
• front
• size
• empty

outputFormat

For each command that returns a value (dequeue, front, size, and empty), print the result on a new line. For the enqueue command, no output is required.

If a command dequeue or front is executed on an empty queue, output None (without quotes).

## sample

9
enqueue 1
enqueue 2
size
front
dequeue
dequeue
dequeue
size
empty

2
1
1
2
None
0
True

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