117-implement-a-stack-

public class Stack {
    
    List<Object> stackList = new List<Object>();
    public void push(Object obj) {
        stackList.add(obj);
    }
    
    public Object pop() {
        return stackList.size() > 0 ? stackList.remove(stackList.size()-1) : null;
    }
    
    public Integer size() {
        return stackList.size();
    }
    
    public Object peek(){
        return stackList.size() > 0 ? stackList.get(stackList.size()-1) : null;
    }
    
    public Boolean isEmpty() {
        return stackList.size() == 0;
    }
}