JavaAlgorithms/ProjectEuler/Problem10.java

package ProjectEuler;

/**
 * The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.
 *
 * <p>Find the sum of all the primes below two million.
 *
 * <p>link: https://projecteuler.net/problem=10
 */
public class Problem10 {
  public static void main(String[] args) {
    long[][] testNumbers = {
      {2000000, 142913828922L},
      {10000, 5736396},
      {5000, 1548136},
      {1000, 76127},
      {10, 17},
      {7, 10}
    };

    for (long[] testNumber : testNumbers) {
      assert solution1(testNumber[0]) == testNumber[1];
    }
  }

  /***
   * Checks if a number is prime or not
   * @param n the number
   * @return {@code true} if {@code n} is prime
   */
  private static boolean isPrime(int n) {
    if (n == 2) {
      return true;
    }
    if (n < 2 || n % 2 == 0) {
      return false;
    }
    for (int i = 3, limit = (int) Math.sqrt(n); i <= limit; i += 2) {
      if (n % i == 0) {
        return false;
      }
    }
    return true;
  }

  private static long solution1(long n) {
    long sum = 0;
    for (int i = 2; i < n; ++i) {
      if (isPrime(i)) {
        sum += i;
      }
    }
    return sum;
  }
}