maxProfit.java

public int maxProfit(int[] a) {
    if (a.length <= 2) {
        return 0;
    }

    int currentMin = a[0];
    int currentMaxProfit = 0;

    for (int i = 1; i < a.length; i++) {
        if (a[i] < currentMin) {
            currentMin = a[i];

        } else {
            int tempProfit = a[i] - currentMin;

            if (tempProfit > currentMaxProfit) {
                currentMaxProfit = tempProfit;
            }
        }
    }

    return currentMaxProfit;
}

primeNumbers.java

//Solution 1: Sieve of Eratosthenes
public int primeNumbers(int a) {

    // create an array where true means the number is prime. All are considered prime at the beginning.
    boolean[] primesList = new boolean[n + 1];
    for (int i = 2; i <= n; i++) {
        primesList[i] = true;
    }

    // mark non-primes
    for (int factor = 2; factor * factor <= n; factor++) {
        // if factor is prime, then mark multiples of factor as nonprime
        if (primesList[factor]) {
            for (int j = factor; factor * j <= n; j++) {
                primesList[factor * j] = false;
            }
        }
    }
    
    // count primes in the list.
    int numberOfPrimes = 0;
    for (int i = 0; i < primesList.length; i++) {
        if (primesList[i] == true) {
            numberOfPrimes++;
        }
    }
    return numberOfPrimes;
}