2 sum?

Determine if there exist two elements in a given array, the sum of which is the given target number.

Solution: HashSet

public boolean existSum(int[] array, int target){
    Set<Integer> set = new HashSet<>();
    for (int num : array){
        if (set.contains(target - num){
        return true;
        }
        set.add(num);
    }
    return false;
}

Time comp: O(N) check every element, O(1) average add/search time

Space comp: O(N) every element add to