読者です 読者をやめる 読者になる 読者になる

ひるあんどんブログ

色々なことに手を出してみるブログ

Letter Queue

In computer science, a queue is a particular kind of data type in which the entities in the collection are kept in order and the principal operations on the collection are the addition of entities to the rear terminal position (enqueue or push), and removal of entities from the front terminal position (dequeue or pop). This makes the queue a First-In-First-Out (FIFO) data structure. In a FIFO data structure, the first element added to the queue will be the first one to be removed. This is equivalent to the requirement that once a new element is added, all elements that were added before have to be removed before the new element can be removed.

We will emulate the queue process with Python. You are given a sequence of commands:

  • "PUSH X" -- enqueue X, where X is a letter in uppercase.
  • "POP" -- dequeue the front position. if the queue is empty, then do nothing.

The queue can only contain letters.

You should process all commands and assemble letters which remain in the queue in one word from the front to the rear of the queue.

Let's look at an example, here’s the sequence of commands:
["PUSH A", "POP", "POP", "PUSH Z", "PUSH D", "PUSH O", "POP", "PUSH T"]
Command Queue Note
PUSH A A Added "A" in the empty queue
POP Removed "A"
POP The queue is empty already
PUSH Z Z
PUSH D ZD
PUSH O ZDO
POP DO
PUSH T DOT The result

Input: A sequence of commands as a list of strings.

Output: The queue remaining as a string.

Example:

letter_queue(["PUSH A", "POP", "POP", "PUSH Z", "PUSH D", "PUSH O", "POP", "PUSH T"]) == "DOT"

letter_queue(["POP", "POP"]) == ""

letter_queue(["PUSH H", "PUSH I"]) == "HI"

letter_queue([]) == ""



How it is used: Queues provide services in computer science, transportation, and operations research where various entities such as data, objects, persons, or events are stored and held to be processed later. In these contexts, the queue performs the function of a buffer.

Precondition:
0 ≤ len(commands) ≤ 30;
all(re.match("\APUSH [A-Z]\Z", c) or re.match("\APOP\Z", c) for c in commands)

def letter_queue(commands):
    result = []
    for command in commands:
        if command.startswith("PUSH"):
            result.append(command[-1])
        elif command == "POP" and len(result) != 0:
            result.pop(0)
        
    return "".join(result)
        
        
    

if __name__ == '__main__':
    #These "asserts" using only for self-checking and not necessary for auto-testing
    assert letter_queue(["PUSH A", "POP", "POP", "PUSH Z", "PUSH D", "PUSH O", "POP", "PUSH T"]) == "DOT", "dot example"
    assert letter_queue(["POP", "POP"]) == "", "Pop, Pop, empty"
    assert letter_queue(["PUSH H", "PUSH I"]) == "HI", "Hi!"
    assert letter_queue([]) == "", "Nothing"

以前同じような課題を解いたので、それと同様にといた。

前回は与えられた文字列(例えば"PUSH A")をstr.split() でわけてから判別していた。

以前の一位の人のコードでstartswithという便利な関数があることを知ったので、今回はそれを使って解いてみた。



pythonはpop()関数で、リストのどこからpopするかを指定することができる。今回はpop(0)で添字0つまり、先頭の要素をpopした。

popは要素がないのに、呼ばれた場合にはエラーを返すのでリストが空でないことは確かめなくてはならないというところで、一回つまづいた。

はやめにエラーだしてくれるのはpythonのいいところな気がします。

さすが、委員長キャラですわ…。

しかし、なぜjoinはstr.join(シーケンス)

ではなく、(シーケンス).join(str) なんだろう。なんか、スッキリしないなぁ…。


一位の人は



def letter_queue(commands):

    import collections

    queue = collections.deque()

    for command in commands:

        if command.startswith("PUSH"):

            queue.append(command[-1])

        elif queue:

            queue.popleft()            

    return "".join(queue)

pythonでキューを実装しようとしたら、collections.dequeを用いると良いみたいです。