Find 2 numbers in an unsorted array equal to a given sum

Question

We need to find pair of numbers in an array whose sum is equal to a given value.

A = {6,4,5,7,9,1,2}

Sum = 10 Then the pairs are - {6,4} ,

Answer 1

    public void printPairsOfNumbers(int[] a, int sum){
    //O(n2)
    for (int i = 0; i < a.length; i++) {
        for (int j = i+1; j < a.length; j++) {
            if(sum - a[i] == a[j]){
                //match..
                System.out.println(a[i]+","+a[j]);
            }
        }
    }

    //O(n) time and O(n) space
    Set cache = new HashSet();
    cache.add(a[0]);
    for (int i = 1; i < a.length; i++) {
        if(cache.contains(sum - a[i])){
            //match//
            System.out.println(a[i]+","+(sum-a[i]));
        }else{
            cache.add(a[i]);
        }
    }

}