Common elements in two lists

Question

I have two ArrayList objects with three integers each. I want to find a way to return the common elements of the two lists. Has anybody an idea how I can achiev

Answer 1

public static  List getCommonElements(
            java.util.Collection a,
            java.util.Collection b
            ) {
        if(a==null && b==null) return new ArrayList<>();
        if(a!=null && a.size()==0) return new ArrayList<>(b);           
        if(b!=null && b.size()==0) return new ArrayList<>(a);
        
        Set set= a instanceof HashSet?(HashSet)a:new HashSet<>(a);
        return b.stream().filter(set::contains).collect(Collectors.toList());
    }

For better time performance, please use HashSet (O(1) look up) instead of List(O(n) look ups)

Time complexity- O(b) Space Complexity- O(a)