ARC112-A B = C

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("5");
            WillReturn.Add("2 6");
            WillReturn.Add("0 0");
            WillReturn.Add("1000000 1000000");
            WillReturn.Add("12345 67890");
            WillReturn.Add("0 1000000");
            //6
            //1
            //0
            //933184801
            //500001500001
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        for (int I = 0; I <= 15; I++) {
            for (int J = I; J <= 15; J++) {
                DeriveCnt(I, J);
            }
        }

        List<string> InputList = GetInputList();

        long[] wkArr = { };
        Action<string> SplitAct = pStr =>
            wkArr = pStr.Split(' ').Select(pX => long.Parse(pX)).ToArray();

        foreach (string EachStr in InputList.Skip(1)) {
            SplitAct(EachStr);
            long L = wkArr[0];
            long R = wkArr[1];

            long N = R - 2 * L + 1;
            if (N < 1) {
                Console.WriteLine(0);
            }
            else {
                Console.WriteLine(N * (N + 1) / 2);
            }
        }
    }

    static void DeriveCnt(long pL, long pR)
    {
        long Answer = 0;
        for (long LoopA = pL; LoopA <= pR; LoopA++) {
            for (long LoopB = pL; LoopB <= pR; LoopB++) {
                for (long LoopC = pL; LoopC <= pR; LoopC++) {
                    if (LoopA == LoopB + LoopC) {
                        Answer++;
                    }
                }
            }
        }
        Console.WriteLine("DeriveCnt({0},{1})={2}", pL, pR, Answer);
    }
}

解説

15以下の組合せで実験すれば、三角数になると分かるので
三角数の一般項である、等差数列の和の公式を使ってます。