1. Iterate from the small elems to large elems
2. If compare each elem on each list,
    If element a is smaller than the element b,
        then increase the index of list contains the 'a' element
        else if two elements are the same
            then add the element to a list if the element is not equal to the last 'intersected element'

    public static List intersect(Integer[] a1, Integer[] a2){
        List list = new ArrayList<>();
        if(a1 != null && a2 != null){
            int len1 = a1.length;
            int len2 = a2.length;
            int i=0, j=0;
            while(i < len1 && j < len2){
                if(a1[i] < a2[j]){
                    i++;
                }else if(a1[i] > a2[j]){
                    j++;
                }else{
                    if(list.size() == 0){
                        list.add(a1[i]);
                    }else{
                        if(list.get(lise.size() - 1) != a1[i])
                            list.add(a1[i]);
                    }
                    i++;
                    j++;
                }
            }
        }
        return list;
    }