题目来自：Leetcode
https://leetcode.com/problems/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.
Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.
class Solution {
public:
int countPrimes(int n) {
if(n<=2)
return 0;
if(n==3)
return 1;
bitset<5000001> b;
b.set();
b.reset(0);
b.reset(1);
int imax=sqrt(n)+1;
int j;
for(int i=2;i<=imax;++i)
{
if(b.test(i)==true)
{
j=i*i;
while(j<n)
{
b.reset(j);
// cout<<"j="<<j<<endl;
j+=i;
}
}
}
b.flip();
return n-b.count();
}
};Count Primes Total Accepted: 831 Total Submissions: 6167
原文：http://blog.csdn.net/zhouyelihua/article/details/45317789