CF 237C (质数区间）

给定区间[a,b] 求l的最小值使[a,b]中任意长度为l的一段包含至少k个Prime

二分l

#include<cstdio>#include<cstring>#include<cstdlib>#include<cmath>#include<algorithm>#include<functional>#include<iostream>using namespace std;#define MAXN (1000000+10)int a[MAXN],tot=0,x,y,k;bool b[MAXN]={0};void work(){b[1]=1;for (int i=2;i<=y;i++){if (!b[i]){tot++;a[tot]=i;}for (int j=1;j<=tot;j++){if (a[j]*i>y) break;b[a[j]*i]=1;if (!i%a[j]) break;}}}bool is_ok(int l){int tot=0;for (int i=x;i<=x+l-1;i++) if (!b[i]) tot++;if (tot<k) return false;for (int j=x+l;j<=y;j++){tot=tot+(!b[j])-(!b[j-l]);if (tot<k) return false;}return true;}int main(){scanf("%d%d%d",&x,&y,&k);work();int l=k,r=y-x+1;if (l>r||!is_ok(r)) {printf("-1\n");return 0;}for (int i=1;i<=60;i++){if (l==r) break;int m=(l+r)>>1;//if (r-l==1) m++;if (is_ok(m)) r=m;else l=m+1;}printf("%d\n",l);//while (1);return 0;}