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  2. Mathematics
  3. Number Theory
  4. Twins
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Twins

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  • chtcholeg1
     + 0 comments

    int solve(long long n, long long m) { // Prime flags in the interval between n and m // It's like Eratosthenes sieve but with offset vector intervalNumbers(m - n + 1, true); if (intervalNumbers.size() < 2) { return 0; } if (n == 1) { intervalNumbers[0] = false; }

    // Eratosthenes sieve from 0 to square root of m
const int lim = sqrt(m);
vector<bool> eratosthenesSieve(lim + 1, true);
eratosthenesSieve[0] = false;
eratosthenesSieve[1] = false;

// Set flags + add primes to vector
vector<long long> primes;
for(int i = 2; i <= lim; ++i) {
    if(eratosthenesSieve[i]) {
        primes.push_back(i);
        for(int j = i * 2; j <= lim; j += i) {
            eratosthenesSieve[j] = false;
            if ( (j >= n) && (j <= m) ) {
                intervalNumbers[j - n] = false;
            }
        }
    }
}

// Set flags for the interval 
for(const long long i: primes) {
    for(int j = max(i * 2, (n + i  - 1) / i * i); j <= m; j += i) {
        intervalNumbers[j - n] = false;            
    }
}

// Calculating
long long int twinCount = 0;
for(size_t i = 0 ; i < intervalNumbers.size() - 2; ++i) {
    if(intervalNumbers[i] && intervalNumbers[i + 2]) {
        ++twinCount;
    }
} 

return twinCount;

    }

  • hannahstephens31
     + 0 comments

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  • CS_2212256_T
     + 0 comments
    import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;

public class Solution {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		int m = sc.nextInt();
		System.out.println(solve(n, m));

		sc.close();
	}

	static int solve(int n, int m) {
		int size = m - n + 1;
		List<Integer> primeDivisors = sievePrimes(size);

		boolean[] primes = new boolean[size];
		Arrays.fill(primes, true);

		for (int primeDivisor : primeDivisors) {
			for (int i = n / primeDivisor * primeDivisor; i <= m; i += primeDivisor) {
				if (i >= n && i > primeDivisor) {
					primes[i - n] = false;
				}
			}
		}

		int result = 0;
		for (int i = 0; i + 2 < primes.length; i++) {
			if (n + i >= 2 && primes[i] && primes[i + 2]) {
				result++;
			}
		}
		return result;
	}

	static List<Integer> sievePrimes(int limit) {
		boolean[] primes = new boolean[limit + 1];
		Arrays.fill(primes, true);

		List<Integer> result = new ArrayList<Integer>();
		for (int i = 2; i < primes.length; i++) {
			if (primes[i]) {
				result.add(i);

				for (long j = (long) i * i; j < primes.length; j += i) {
					primes[(int) j] = false;
				}
			}
		}
		return result;
	}
}

  • fivenightsatfre2
     + 0 comments

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  • specificshel
     + 0 comments

    good nice.