原题链接:http://acm.nefu.edu.cn/JudgeOnline/problemshow.php?problem_id=117
分析：设数为N,则其位数为(int)lg(N)+1;log(10)=ln(10);
素数定理：令Pn表示<n的素数个数，则当n较大时有Pn~nln(n).
1 #include<iostream> 2 #include<cstdio> 3 #include<cstring> 4 #include<cmath> 5 #include<algorithm> 6 using namespace std; 7 int main() 8 { 9 int n; 10 double e=2.71828; 11 while(cin>>n) 12 { 13 double ans=double(n-log10(n)-log10(log(10))); 14 cout<<(int)ans+1<<endl; 15 } 16 return 0; 17 }