💡 ArrayList로 스택 구현 public class Implementation_Stack { private ArrayList listStack = new ArrayList(); public void push(Integer data) { listStack.add(data); } public Integer pop() { if (listStack.size() == 0) { return null; } else { return listStack.remove(listStack.size()-1); } } public int size() { return listStack.size(); } public Integer peek() { if (listStack.size() == 0) { return null; } el..