ABC177-C Sum of product of pairs

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("1 2 3");
            //11
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("4");
            WillReturn.Add("141421356 17320508 22360679 244949");
            //437235829
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

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

        long ModVal = 1000000000 + 7;

        long[] AArr = InputList[1].Split(' ').Select(X => long.Parse(X)).ToArray();
        long wkSum = 0;
        foreach (long EachInt in AArr) {
            wkSum += EachInt;
            wkSum %= ModVal;
        }

        long Answer = 0;
        for (long I = 0; I <= AArr.GetUpperBound(0) - 1; I++) {
            wkSum -= AArr[I];
            wkSum += ModVal;
            wkSum %= ModVal;

            Answer += AArr[I] * wkSum;
            Answer %= ModVal;
        }
        Console.WriteLine(Answer);
    }
}

解説

A,B,C,Dの4つとしたら
Answer = A*B + A*C + A*D + B*C + B*D + C*D となり、変形すると
Answer = A*(B+C+D) + B*(C+D) + C*D なので、
最初に、A+B+C+Dの値を計算してます。