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// http://ja.wikipedia.org/wiki/モンティ・ホール問題

#include <iostream>
#include <cstdlib>

using namespace std;

int
main(int, char**)
{
    const int N = 100000;
    const int n = 3;    // # of doors
    int p1 = 0;
    int p2 = 0;
    for (int k = 0; k < N; k++) {
        bool x[n] = {};

        int hit = rand() % n;
        x[hit] = true;

        // player's 1st choice
        int a1 = rand() % n;

        // monty's suggestion
        int hint;
        for (;;) {
            hint = rand() % n;
            if (hint != a1 && hint != hit)
                break;
        }

        // player's 2nd choice
        int a2;
        for (;;) {
            a2 = rand() % n;
            if (a2 != a1 && a2 != hint)
                break;
        }

        if (a1 == hit)
            p1++;
        if (a2 == hit)
            p2++;
    }

    cout << "p1 " << (1.0 * p1 / N) << endl;
    cout << "p2 " << (1.0 * p2 / N) << endl;

    return 0;
}

% c++ monty.cc
% ./a.out
p1 0.33477
p2 0.66523
%