public int top(){
        if(size == 0){
            throw new IllegalArgumentException();
        }
        return head.data;
    }
http://www.geeksforgeeks.org/design-a-stack-with-find-middle-operation/
Test cases:
Delete middle with 1,2,3 element
Pop with 1,2,3 elements
Delete or pop when empty stack
src/com/interview/linklist/StackWithLinkListMiddleOperation.java