Java Cloud Conference (2018) Caffeine Challenge. More about Caffeine : https://youtu.be/Xl1XBJLfIDU

DuplicateSumTest.java

public class DuplicateSumTest {
    @Test
    public void sumNonDuplicates() {
        Assert.assertEquals("", 3, sumNonDuplicates(new int[]{1, 2}));
    }

    @Test
    public void sumDuplicates() {
        Assert.assertEquals("", 3, sumNonDuplicates(new int[]{2, 1, 2, 1}));
    }

    public int sumNonDuplicates(int[] input) {
        //don't create another array or collection
        return 0;
    }
}

return IntStream.of(input).distinct().sum();

return 3;

The Stream version will still create collections under the covers.

public int sumNonDuplicates(int[] input) {
    int sum = 0;
    int current = 0;
    Arrays.sort(input);
    for (int i = 0; i < input.length; i++) {
        int value = input[i];
        if (value != current) {
            sum += value;
            current = value;
        }
    }
    return sum;
}