1752. 選擇

soj.me/1725

#include <iostream>#include <cstring>using namespace std;int p[5001],arr[1000001],n;int prime(){memset(arr,0,sizeof(arr));int k=1;for(int i=2;i<1000000;i++){if(arr[i]==0){p[k++]=i;for(int j=2;i*j<1000000;j++)arr[i*j]=1;}}return k;}int bsearch(int min,int max){int mid=(min+max)/2;while(min<=max){mid=(min+max)/2;if(p[mid]>n)max=mid-1;else if(p[mid]<n)min=mid+1;else return mid;}return mid;}int main(){int len=prime();while(cin>>n&&n!=-1){int x=bsearch(1,len);cout<<x<<endl;}return 0;}