筛法求素数
class Solution {
public:
int countPrimes(int n) {
int ans=0;
vector<int> A(n,1);
for(long long i=2;i<n;++i)
if(A[i]){
++ans;
for(long long k=i*i;k<n;k+=i) A[k]=0;
}
return ans;
}
};