VIJOS 1889 天真的因数分解

 1 #include<algorithm>
 2 #include<cstdio>
 3 #include<cmath>
 4 #include<cstring>
 5 #include<iostream>
 6 #define N 200000
 7 #define ll long long
 8 int mul[N+10],mark[N+10],p[N+10];
 9 ll K;
10 void init(){
11     mul[1]=1;
12     for (int i=2;i<=N;i++){
13         if (!mark[i]){
14             p[++p[0]]=i;
15             mul[i]=1;
16         }
17         for (int j=1;j<=p[0]&&p[j]*i<=N;j++){
18             mark[i*p[j]]=1;
19             if (i%p[j]) mul[i*p[j]]=-mul[i];
20             else{
21                 mul[i*p[j]]=0;
22                 break;
23             }
24         }
25     }
26 }
27 ll cal(ll x){
28     ll sum=0,t=sqrt(x);
29     for (ll i=2;i<=t;i++){
30         sum+=x/(i*i)*mul[i];
31     }
32     return sum;
33 }
34 int main(){
35     scanf("%lld",&K);
36     init();
37     ll l=K,r=25505460948LL,ans=0;
38     while (l<=r){
39         ll mid=(l+r)>>1;
40         if (cal(mid)>=K) ans=mid,r=mid-1;
41         else l=mid+1;
42     }
43     printf("%lld\n",ans);
44 }