HDU 5901 Count primes
题目链接:
http://acm.hdu.edu.cn/showproblem.php?pid=5901
Problem Description
Easy question! Calculate how many primes between [1...n]!
Input
Each line contain one integer n(1 <= n <=
1e11).Process to end of file.
Output
For each case, output the number of primes in interval
[1...n]
Sample Input
2
3
10
Sample Output
1
2
4
Hint:
题意:
比较简单,这里就不说了。
题解:
自己开始的时候数据范围没有看清楚,以为是水题,结果就GG了。
网上大神的代码,自己好好学习学习。
代码:
#include <bits/stdc++.h> #define ll long long using namespace std; ll f[340000],g[340000],n; void init(){ ll i,j,m; for(m=1;m*m<=n;++m)f[m]=n/m-1; for(i=1;i<=m;++i)g[i]=i-1; for(i=2;i<=m;++i){ if(g[i]==g[i-1])continue; for(j=1;j<=min(m-1,n/i/i);++j){ if(i*j<m)f[j]-=f[i*j]-g[i-1]; else f[j]-=g[n/i/j]-g[i-1]; } for(j=m;j>=i*i;--j)g[j]-=g[j/i]-g[i-1]; } } int main(){ while(scanf("%I64d",&n)!=EOF){ init(); cout<<f[1]<<endl; } return 0; }