// CoMP1006 W13
// Stack class 

public class Stack{
  // we'll use a partially filled array to store our stack
  private Object[] data; 
  private int      size; 
  
  // constructors
  public Stack(){data = new Object[10];  size = 0;}
  public Stack(int s){data = new Object[s]; size = 0;}
  
  
  // methods
  public int size(){ return size;}
  
  public void push(Object item){
    if(size >= data.length - 1){
       Object[] newData = new Object[data.length * 2];
       System.arraycopy(data,0,newData,0,size);
       data = newData;
    }
    data[size] = item;
    size += 1;
  }
  

  public Object pop(){
    //todo: check if size is too small and reduce the size
    //      of the stack
    if(size == 0){
      return null;
    }else{
      size -= 1;
      Object item = data[size];
      data[size] = null;
      return item;
    } 
  }
  
  public Object peek(){
    if(size==0){
      // handle special case of empty stack
      return null;
    }else{
      // non-empty case of stack
      return data[size-1];
    }
  }
}