@hassanmohsin Hey Hassan, my idea is quite similar to yours. While when I was trying to add two elements of stack, compiling was failed. It warns me that Integer cannot be converted into int to do mathematical operations. i.e. in your code case, item1 is int, while bucket.pop() returns an Integer object, thus you cannot make them equal =, as well as add item1 and item2 up. Would you mind explaining this to me? Thanks.