Count Primes

Description:

Count the number of prime numbers less than a non-negative number, n

Hint: The number n could be in the order of 100,000 to 5,000,000.

C++实现代码：

#include<new>

#include<iostream>

#include<cmath>

#include<cstring>

using namespace std;



class Solution {

public:

    int countPrimes(int n) {

        bool *arr=new bool[n+1];

        memset(arr,true,n);

        int count=0;

        for(int i=2;i*i<=n;++i)

        {

            if(arr[i])

            {

                int j=i;

                while(j*i<=n)

                {

                    arr[i*j]=0;

                    ++j;

                }

            }

        }

        for(int i=2;i<n;++i)

        {

            if(arr[i])

                count++;

        }

        return count;

    }

};

int main()

{

    Solution s;

    cout<<s.countPrimes(499979)<<endl;

}