How can I find two elements in an array that sum to k

Question

Suppose you are given an array of unsorted integers as

A = {3,4,5,1,4,2}

Input : 6 Output : {5,1}, {4,2}

Answer 1

As with your other question, O(log n) is impossible, since you have to examine the entire array. But O(n) is more or less possible.

If your range of possible integers is relatively small — that is, if it's within a constant factor of n — then you can write:

final boolean[] seen = new boolean[max - min + 1];
for(final int a : A)
{
    if(seen[input - a - min])
        System.out.println("{" + (input - a) + "," + a + "}");
    seen[a - min] = true;
}

If not, you can do the same thing, but using a HashSet instead of an array:

final Set seen = new HashSet();
for(final int a : A)
{
    if(seen.contains(input - a))
        System.out.println("{" + (input - a) + "," + a + "}");
    seen.add(a);
}

but that will not have guaranteed O(n) time.