백준 3896번 소수 사이 수열

문제 링크입니다: https://www.acmicpc.net/problem/3896

에라토스테네스의 체를 이용하여 푸는 문제였습니다.

주의할 점은 소수 사이 수열의 길이는 (소수들의 개수 + 1)이라는 것입니다.

#include <iostream>

#include <vector>

using namespace std;

 

const int MAX = 1299709 + 1;

 

int minFactor[MAX];

 

void eratosthenes(void)

{

        minFactor[0] = minFactor[1] = -1;

        for (int i = 2; i < MAX; i++)

                 minFactor[i] = i;

 

        for (int i = 2; i*i < MAX; i++)

                 if (minFactor[i] == i)

                         for (int j = i * i; j < MAX; j += i)

                                 if (minFactor[j] == j)

                                          minFactor[j] = i;

}

 

int main(void)

{

        ios_base::sync_with_stdio(0);

        cin.tie(0);

        int T;

        cin >> T;

 

        eratosthenes();

        for (int t = 0; t < T; t++)

        {

                 int K;

                 cin >> K;

 

                 if (minFactor[K] == K)

                         cout << 0 << "\n";

                 else

                 {

                         int result = 1;

                         int temp = K + 1;

                         while (1)

                         {

                                 if (minFactor[temp] == temp)

                                          break;

 

                                 temp++;

                                 result++;

                         }

                         temp = K - 1;

                         while (1)

                         {

                                 if (minFactor[temp] == temp)

                                          break;

 

                                 temp--;

                                 result++;

                         }

                         //중요

                         cout << result + 1 << "\n";

                 }

        }

        return 0;

}

개발환경:Visual Studio 2017

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