E8本(数学) 097 Primes in an Interval

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("21 40");
            //4
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("101 130");
            //6
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("1 100");
            //25
        }
        else if (InputPattern == "Input4") {
            WillReturn.Add("217 217");
            //0
        }
        else if (InputPattern == "Input5") {
            WillReturn.Add("999999500000 1000000000000");
            //18228
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

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

        long L = wkArr[0];
        long R = wkArr[1];

        long SqrtVal = 1;
        while (SqrtVal * SqrtVal <= R) {
            SqrtVal++;
        }
        Eratosthenes(SqrtVal);

        var NonPriceSet = new HashSet<long>();
        foreach (long EachPrime in mSosuuArr) {
            long CurrVal = L - (L % EachPrime);
            while (CurrVal <= R) {
                if (Array.BinarySearch(mSosuuArr, CurrVal) < 0) {
                    if (L <= CurrVal && CurrVal <= R) {
                        NonPriceSet.Add(CurrVal);
                    }
                }
                CurrVal += EachPrime;
            }
        }
        long Answer = R - L + 1 - NonPriceSet.Count;
        if (L == 1) Answer--;
        Console.WriteLine(Answer);
    }

    static long[] mSosuuArr;

    // エラトステネスの篩
    static void Eratosthenes(long pJyougen)
    {
        bool[] IsSosuuArr = new bool[pJyougen + 1];

        for (long I = 2; I <= IsSosuuArr.GetUpperBound(0); I++) {
            IsSosuuArr[I] = true;
        }
        for (long I = 2; I * I <= IsSosuuArr.GetUpperBound(0); I++) {
            if (IsSosuuArr[I]) {
                for (long J = I * 2; J <= IsSosuuArr.GetUpperBound(0); J += I) {
                    IsSosuuArr[J] = false;
                }
            }
        }
        var SosuuList = new List<long>();
        for (long I = 2; I <= IsSosuuArr.GetUpperBound(0); I++)
            if (IsSosuuArr[I]) SosuuList.Add(I);

        mSosuuArr = SosuuList.ToArray();
    }
}

解説

区間篩で解いてます。