Euler Problem 46 solution

Time (s): ~0.030

package margusmartseppcode.From_40_to_49;

public class Problem_46 {
 /** Call only if: (n>0&&(i%2==0||i%3==0||i%5==0||i%7==0)) */
 static boolean isPrimeS(final int n) {
  final int r = (int) Math.floor(Math.sqrt(n));
  for (int f = 5; f <= r; f += 6)
   if (n % f == 0 || n % (f + 2) == 0) return false;
  return true;
 }

 /** Will return array with first nr primes */
 static int[] getPrimes(int nr) {
  if (nr < 0) return new int[0];

  int amount = 0;
  int result[] = new int[nr];

  if (nr > 0) result[amount++] = 2;
  if (nr > 1) result[amount++] = 3;
  if (nr > 2) result[amount++] = 5;
  if (nr > 3) result[amount++] = 7;
  if (nr <= 4) return result;

  for (int i = 9;; i += 2) {
   if (i % 3 == 0 || i % 5 == 0 || i % 7 == 0) continue;
   if (isPrimeS(i)) {
    result[amount++] = i;
    if (amount >= nr) break;
   }
  }
  return result;
 }

 public static void main(String[] args) {
  double tmp = 0;
  int i = 0, num = 3, p[] = getPrimes(1000);

  for (i = 0; !(p[i] > num); i++, tmp = Math.sqrt((num - p[i]) / 2))
   if (tmp == Math.ceil(tmp)) {
    i = 0;
    num += 2;
   }

  System.out.println(num);
 }
}

Time (s): ~0.038

package margusmartseppcode.From_40_to_49;

import java.util.ArrayList;
import java.util.BitSet;

public class Problem_46 {
 /** Sieve of Eratosthenes */
 static Integer[] getSoE(int max) {
  if (max < 1) return new Integer[0];

  BitSet sieve = new BitSet(max / 2);
  ArrayList<Integer> list = new ArrayList<integer>();
  if (max > 1) list.add(2);
  if (max > 2) list.add(3);

  for (int i = 5, f = 1; i <= max; i += 3 - f, f = -f)
   if (sieve.get(i >> 1) == false) {
    for (int add, j = i + (add = i << 1); j < max; j += add)
     sieve.set(j >> 1, true);
    list.add(i);
   }

  return list.toArray(new Integer[0]);
 }

 public static void main(String[] args) {
  double tmp = 0;
  int i = 0, num = 3;
  Integer p[] = getSoE(10000);

  for (i = 0; !(p[i] > num); i++, tmp = Math.sqrt((num - p[i]) / 2))
   if (tmp == Math.ceil(tmp)) {
    i = 0;
    num += 2;
   }

  System.out.println(num);
 }
}

Euler Problem 10 solution

Time (s): ~0.097

package margusmartseppcode.From_10_to_19;

import java.util.BitSet;

public class Problem_10 {
 /** Sieve of Eratosthenes sum */
 static long getSoEsum(int max) {
  if (max < 1)
   return 0;
  BitSet sieve = new BitSet(max / 2);
  long sum = (max > 1 ? 2 + (max > 2 ? 3 : 0) : 0);

  for (int i = 5, f = 1; i <= max; i += 3 - f, f = -f)
   if (sieve.get(i >> 1) == false) {
    int add = i << 1;
    for (int j = i + add; j < max; j += add)
     sieve.set(j >> 1, true);
    sum += i;
   }

  return sum;
 }

 public static void main(String[] args) {
  System.out.println(getSoEsum(2000000));
 }
}

Euler Problem 7 solution

Time (s): ~0.011

package margusmartseppcode.From_1_to_9;

import java.util.BitSet;

public class Problem_7 {
 static long getSoExnt(int max, int nr) {
  if (max < 1 || nr < 1 || max < nr)
   return -1;
  if (nr < 4)
   return (nr == 1 ? 2 : (nr == 2 ? 3 : 5));
  BitSet sieve = new BitSet(max / 2);

  for (int i = 5, f = 1, c = 2; i <= max; i += 3 - f, f = -f)
   if (sieve.get(i >> 1) == false) {
    int add = i << 1;
    for (int j = i + add; j < max; j += add)
     sieve.set(j >> 1, true);
    if (++c >= nr)
     return i;
   }

  return -1;
 }

 public static void main(String[] args) {
  System.out.println(getSoExnt(120000, 10001));
 }
}