输出N以内素数的方法

最近有个朋友问我"输出N以内素数的方法", 我想网上一定有很多方法, 而且代码有可能很雷同. 不过还是自己写了一段.

贪心法明显要快很多. 

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

namespace ConsoleApplication2
{
    
    class Program
    {
        //普通方法
        public static List<int> GetPrimeList(int maxNum)
        {
            List<int> primeList = new List<int>();
            primeList.Add(2);
            for (int onCheck = 3; onCheck <= maxNum; ++onCheck)
            {
                bool isPrime = true;
                for (int checker = 2; checker <= Math.Sqrt(onCheck); ++checker)
                {
                    if (0 == onCheck % checker)
                    {
                        isPrime = false;
                        break;
                    }
                }
                if (true == isPrime)
                {
                    primeList.Add(onCheck);
                }
            }

            return primeList;
        }

        //贪心法
        public static List<int> GetPrimeList(int maxNum, bool useGreedy)
        {
            List<int> primeList = new List<int>();
            if (true == useGreedy)
            {
                bool[] map = new bool[maxNum + 1];
                for (int i = 2; i <= maxNum; ++i)
                {
                    map[i] = true;
                }
                for (int i = 2; i <= Math.Sqrt(maxNum); ++i)
                {
                    if (true == map[i])
                    {
                        for (int j = i*i; j <= maxNum; j += i)
                        {
                            map[j] = false;
                        }
                    }
                }
                for (int i = 2; i <= maxNum; ++i)
                {
                    if (true == map[i])
                    {
                        primeList.Add(i);
                    }                    
                }
            }
            else
            {
                primeList = GetPrimeList(maxNum);
            }

            return primeList;
        }
        static void Main(string[] args)
        {
            int timeStart = DateTime.UtcNow.Minute * 60 + DateTime.UtcNow.Second;
            List<int> primes = GetPrimeList(100000000,true);
            int timeEnd = DateTime.UtcNow.Minute * 60 + DateTime.UtcNow.Second;
            //foreach (int item in primes)
            //{
            //    Console.Write("{0},", item);
            //}
            //Console.Write("\n");
            Console.WriteLine("primes cont: {0}, used {1} second",primes.Count, timeEnd-timeStart);
            Console.Read();
        }
    }
}