poj 3518 Prime Gap

Prime Gap

#include<cmath>
#include<cstdio>
#include<algorithm>
#include<iostream>
#include<string>
#include<cstring>
#include<vector>
using namespace std;
#define max 100000
int prime[max+5];
//bool vis[max+5];
void get_prime(int n){
    //memset(vis,false,sizeof(vis));
    prime[0]=2;
    prime[1]=2;
    int i=3,index=2;
    for(;index<=n;i+=2){
        //cout<<i<<endl;
        //cout<<prime[index-1]<<endl;
        int half=sqrt(i*1.0)+1;
        int j=1;
        while(j<index&&half>prime[j]){//不知道为什么j<index可以去掉。去掉也是对的。
            //cout<<i<<" "<<j<<" "<<prime[j]<<endl;
            if(i%prime[j]==0){
                break;
            }
            j++;
        }
        if(half<=prime[j]){
            prime[index++]=i;
        }
    }
    //cout<<prime[index-1]<<endl;
}
int main(){
    int n;
    get_prime(max);
    while(cin>>n&&n){
        int i=1;
        while(prime[i]<n){
            i++;
        }
        if(prime[i]==n){
            cout<<0<<endl;
        }
        else{
            cout<<prime[i]-prime[i-1]<<endl;
        }
    }
    return 0;
}

 

Time Limit: 5000MS		Memory Limit: 65536K
Total Submissions: 8766		Accepted: 5148

Description

The sequence of n − 1 consecutive composite numbers (positive integers that are not prime and not equal to 1) lying between two successive prime numbers p and p + n is called a prime gap of length n. For example, ‹24, 25, 26, 27, 28› between 23 and 29 is a prime gap of length 6.

Your mission is to write a program to calculate, for a given positive integer k, the length of the prime gap that contains k. For convenience, the length is considered 0 in case no prime gap contains k.

Input

The input is a sequence of lines each of which contains a single positive integer. Each positive integer is greater than 1 and less than or equal to the 100000th prime number, which is 1299709. The end of the input is indicated by a line containing a single zero.

Output

The output should be composed of lines each of which contains a single non-negative integer. It is the length of the prime gap that contains the corresponding positive integer in the input if it is a composite number, or 0 otherwise. No other characters should occur in the output.

Sample Input

10
11
27
2
492170
0

Sample Output

4
0
6
0
114

Source

Japan 2007

 

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