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アルゴリズムいちねんせい AGC010 B
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「AtCoder Grand Contest 010」の問題B「Boxes」のアルゴリズム初心者向けの解説です。
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アルゴリズムいちねんせい AGC010 B
1.
アルゴリズムいちねんせい AGC010 B問題の解き方 @takepan
2.
AGC010 問題B • 問題を眺めます。 •
ピンときた人は、「アルゴリズムいちねんせい」卒業です。このファイルを閉じ てください。 • ピンと来なかった人は、次のページを見てみましょう。
3.
AGC010 問題B 上記の状態から、箱2を「君に決めた!」と言いつつ、箱3から1個、箱4から2個、箱5から3個、 その次の箱1から4個、箱2から5個、石を取り出します。 • 入力例1の解説 箱1
箱2 箱3 箱4 箱5 4 5 1 2 3 箱1 箱2 箱3 箱4 箱5 4 5 1 2 3 -4 -5 -1 -2 -3 0 0 0 0 0 すべての箱の中の石を取り除けたので、「YES」を出力します。
4.
入力例1では説明文がついていましたが、ここには解説がありません。いきなり大海原に投げ 出された気分ですね。 入力例1では4,5と1,2,3となんとなく数字が単純に増えていましたが、今回は減っている箇 所があります。(赤の矢印のところ) AGC010 問題B • 入力例2の解説 箱1
箱2 箱3 箱4 箱5 6 9 12 10 8 箱1 箱2 箱3 箱4 箱5 6 9 12 10 8
5.
円構造であることを分かりやすくするため、箱5のあとに箱1を追加しました。 こう見ると、3箇所で減っているのが分かります。しかも綺麗に2ずつです。 どの箱からかは分かりませんが、そこを起点に1,2,3,4,5個を取り出していきます。 途中で取る個数が減るのは、5→1のところです。そこに当てはめて考えてみます。 AGC010 問題B • 入力例2の解説 箱1
箱2 箱3 箱4 箱5 箱1 6 9 12 10 8 6
6.
上記の表のように、石を取っていくとすべての箱の中の石を取り除くことができます。 なので、答えは「YES」です。 AGC010 問題B • 入力例2の解説 箱1
箱2 箱3 箱4 箱5 箱1 6 9 12 10 8 6 -3 -4 -5 -1 -2 -3 -2 -3 -4 -5 -1 -2 -1 -2 -3 -4 -5 -1 0 0 0 0 0 0
7.
箱5個のときは、取り出す石の数は1、2、3、4、5なので、1回に合計15個の石を取り出すことに なります。 箱の合計の石の数の合計(6+9+12+10+8)は45個なので、取り出す操作は3回行います。 そうなると、箱の中の石の数の並びで、「減る箇所」が「のべ3回」発生することになります。 「のべ」と書いたのは、3回とも同じ箇所で減るケースがあるからです。 AGC010 問題B • 入力例2の解説
8.
勝手に入力例4を作りました。減る箇所は1箇所です。 入力例2では減る箇所の減る数は2でしたが、今回は12も減っています。なぜでしょうか? これは、隣り合う箱の数の差は、1回の操作では1箇所以外は「右の箱」引く「左の箱」が1で、 1箇所だけマイナスN-1(今回はNが5なので-4)だからです。 入力例2の3回の操作では、増える回数が2、減る回数が1で「+1」「+1」「-4」で合計-2でした。 これが重なると「-4」「-4」「-4」で合計-12になる訳ですね。 AGC010 問題B • 入力例4 箱1
箱2 箱3 箱4 箱5 箱1 3 6 9 12 15 3 -1 -2 -3 -4 -5 -1 -1 -2 -3 -4 -5 -1 -1 -2 -3 -4 -5 -1 0 0 0 0 0 0
9.
※「取り出す箇所の切れ目」は、今までのページに出てきた、矢印の部分のことです。 AGC010 問題B • まとめると •
1度も減らない箇所は、取り出す操作の回数だけ増える • 上記に当てはまらない箇所は、「取り出す箇所の切れ目」がある。 「取り出す回数に足りない数」÷(N-1) で、何回切れ目になっているか分かる
10.
AGC010 問題B 2~4行目はデータの読み込み部分です。 PHPの「おまじない」です。 石の総計を計算し、Nによって定まる「1回の操 作で取り出す石の数」を計算します。 石の総数を「1回の操作で取り出す石の数」で 割り、整数でなければ「NO」を出力して終了し ます。 • 実装
11.
AGC010 問題B • 実装 配列の最後に1番めの箱の石の数を追加して います。 一度も切れ目がない箇所では、$diffは0になり ます。 あ 今見返してみると、27行目と29行目は1行で済みますね。 ($kaisuuの意味がない……) $diffがマイナスで$cutもマイナスで数えている ため、
$round==$cut ではなく、 $round==-$cut になっています。 $round+$cut==0 の方が分かりやすい かもしれませんね。
12.
AGC010 問題B • おまけ 実は、コンテストのときに提出したプログラムに はミスがありました。具体的には、27行目の 「floor()」(床関数による整数化)の部分があり ませんでした。 その場合、例えばN=4で取り出し操作が3回、1 番目と3番目の箱の前で切れ目が「1.5回ずつ」 ある場合に「YES」と出力してしまいます。 TopCoderやcodeForcesならばたぶん撃墜され ていたはずで、これが正解にしてもらえるのは、 AtCoderならでは、だったかもしれません。 4 6
9 6 9
13.
おしまい
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