java - Determine if array contains two elements which equal a certain sum?

Question

// Checks whether the array contains two elements whose sum is s.
// Input: A list of numbers and an integer s
// Output: return True if the answer is yes, else return False

public static boolean calvalue (int[] numbers, int s){
for (int i=0; i< numbers.length; i++){
    for (int j=i+1; j<numbers.length;j++){
        if (numbers[i] < s){
            if (numbers[i]+numbers[j] == s){
                return true;
                }
            }
        }
    }
    return false;
}

Answer 1

This can be achieved in O(n).

1. Create a hash-backed set out of your list, such that it contains all elements of the list. This takes O(n).
2. Walk through each element n of your list, calculate s-n = d, and check for the presence of d in the set. If d is present, then n+d = s, so return true. If you pass through the list without finding an appropriate d, return false. This is achieved in a single pass through your list, with each lookup taking O(1), so this step also takes O(n).