ARC107-A Simple Math

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("1 2 3");
            //18
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("1000000000 987654321 123456789");
            //951633476
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    const long Hou = 998244353;

    static void Main()
    {
        List<string> InputList = GetInputList();
        long[] wkArr = InputList[0].Split(' ').Select(pX => long.Parse(pX)).ToArray();

        long A = wkArr[0];
        long B = wkArr[1];
        long C = wkArr[2];

        Func<long, long> DeriveSumFunc = pLimitVal =>
        {
            return pLimitVal * (pLimitVal + 1) / 2;
        };

        long SumA = DeriveSumFunc(A) % Hou;
        long SumB = DeriveSumFunc(B) % Hou;
        long SumC = DeriveSumFunc(C) % Hou;

        long Answer = SumA;
        Answer *= SumB;
        Answer %= Hou;
        Answer *= SumC;
        Answer %= Hou;

        Console.WriteLine(Answer);
    }
}

解説

因数分解すると
等差数列Aの総和 * 等差数列Bの総和 * 等差数列Cの総和
になるので、この値を計算してます。