O(n log(n)) algorithm that checks if sum of 2 numbers in a int[] = given number

Question

I am supposed to create a O(n log(n)) algorithm that checks if sum of 2 numbers in a int[] == given number.

eg. Given [1,4,7,2,3,4] there will be a sum

Answer 1

Similar to @user384706, however you can do this with in O(n).

What they say is the following: S=[1,4,7,2,3,4]

Add these to a HashSet, ideally TIntHashSet (but the time complexity is the same)

int total = 9;
Integer[] S = {1, 4, 7, 2, 3, 4, 6};
Set set = new HashSet(Arrays.asList(S));
for (int i : set)
    if (set.contains(total - i))
        System.out.println(i + " + " + (total - i) + " = " + total);

prints

2 + 7 = 9
3 + 6 = 9
6 + 3 = 9
7 + 2 = 9