min_25筛题目总结

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 typedef long long int ll;
 4 const ll maxn=1E6+5;
 5 ll n,prime[maxn],size,sqr,back[maxn],m,g[maxn],id1[maxn],id2[maxn];
 6 bool vis[maxn];
 7 void init(ll n)
 8 {
 9     for(int i=2;i<=n;++i)
10     {
11         if(!vis[i])prime[++size]=i;
12         for(int j=1;j<=size&&i*prime[j]<=n;++j)
13         {
14             vis[i*prime[j]]=1;
15             if(i%prime[j]==0)break;
16         }
17     }
18 }
19 void put(ll x,int y)
20 {
21     if(x<=sqr)id1[x]=y;
22     else id2[n/x]=y;
23 }
24 int where(ll x)
25 {
26     if(x<=sqr)return id1[x];
27     else return id2[n/x];
28 }
29 int main()
30 {
31     ios::sync_with_stdio(false);
32     cin>>n;
33     sqr=sqrt(n)+1;
34     init(sqr);
35     for(ll i=1,j;i<=n;i=j+1)
36     {
37         back[++m]=n/i;
38         j=n/back[m];
39         put(n/i,m);
40         g[m]=back[m]-1;
41     }
42     for(int j=1;j<=size;++j)
43     {
44         ll limit=prime[j]*prime[j];
45         for(int i=1;back[i]>=limit;++i)
46         {
47             int k=where(back[i]/prime[j]);
48             g[i]+=j-1-g[k];
49         }
50     }
51     cout<<g[1]<<endl;
52     return 0;
53 }