プログラミングコンテスト系の標準入力からデータを受け取るコード

Main.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {

    public static void main(String[] args) {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String input = null;
        try {
            input = new String(in.readLine());
        } catch (IOException e) {
            e.printStackTrace();
        }

        String[] strs = input.split(" ");
        n = Integer.parseInt(strs[0]);
        k = Integer.parseInt(strs[1]);

        if (k >= n) {
            System.out.println("1.000000000000");
            return;
        }

        make_perm(0, new int[n], new boolean[n + 1]);


        System.out.println(findCount / (double) count);
    }

    public static int k = 0;
    public static int n = 0;
    public static int count = 0;
    public static int findCount = 0;

    public static void count_perm(int[] perm){
        count++;
        int maxExp = 0;
        int eatMax = 0;
        int eatCount = 0;

        for (int j = 0; j < perm.length; j++) {

            if (0 != j &&
                    perm[j] > maxExp &&
                    eatCount < k) {
                eatMax = perm[j];
                eatCount++;
            }

            if (perm[j] > maxExp) {
                maxExp = perm[j];
            }
        }

        if (eatMax == n) {

            findCount++;
        }
    }

    public static void make_perm(int n, int[] perm, boolean[] flag){
        if(n == perm.length){
            count_perm(perm);
        } else {
            for(int i = 1; i <= perm.length; i++){
                if(flag[i]) continue;
                perm[n] = i;
                flag[i] = true;
                make_perm(n + 1, perm, flag);
                flag[i] = false;
            }
        }
    }
}