Educational DP Contest I Coins

問題へのリンク

C#のソース

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("3");
            WillReturn.Add("0.30 0.60 0.80");
            //0.612
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("1");
            WillReturn.Add("0.50");
            //0.5
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("5");
            WillReturn.Add("0.42 0.01 0.42 0.99 0.42");
            //0.3821815872
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();

        int N = int.Parse(InputList[0]);
        int UB = N;

        double[] PArr = InputList[1].Split(' ').Select(pX => double.Parse(pX)).ToArray();

        // 確率[表の枚数]なDP表
        double[] PrevDP = new double[UB + 1];
        PrevDP[0] = 1;

        foreach (double EachProb in PArr) {
            double[] CurrDP = new double[UB + 1];
            for (int I = 0; I <= UB; I++) {
                if (PrevDP[I] == 0) continue;

                Action<int, double> UpdateAct = (pNewI, pProb) =>
                {
                    CurrDP[pNewI] += PrevDP[I] * pProb;
                };
                UpdateAct(I + 1, EachProb);
                UpdateAct(I, 1 - EachProb);
            }
            PrevDP = CurrDP;
        }

        double Answer = 0;
        for (int I = 0; I <= UB; I++) {
            if (I > N - I) {
                Answer += PrevDP[I];
            }
        }
        Console.WriteLine(Answer);
    }
}

解説

配るDPで、確率の加法定理と乗法定理を使ってます。