// Michael Otteneder, 1120886; Max Horejs, 0505847
public class ArrayStack implements Stack {

 private Object arr[];
 private int i=0;
 public ArrayStack(){
  arr = new Object[8];
 }
 @Override
 public void push(Object item) {
  if(i < arr.length){
   arr[i] = item;
   i++;
  }
  else{
   Object new_arr[] = new Object[arr.length*2];
   for(int j=0; j<arr.length; j++){
    new_arr[j] = arr[j];
   }
   new_arr[i] = item;
   arr = new_arr;
   i++;
  }
  
 }

 @Override
 public Object pop() {
 if(!this.isEmpty()){
  i--; 
  Object o = arr[i];
  return o;
 }
 else{
  return null;
 }
 }

 @Override
 public Object peek() {
  if(!this.isEmpty()){
   return arr[i-1];
  }
  else{
   return null;
  }
 }

 @Override
 public boolean isEmpty() {
  return i<1;
 }
 public String toString(){
  StringBuffer sb = new StringBuffer();
    for(int i = 0; i<this.i; i++){
     sb.append(" " + arr[i]);
    }
     return sb.toString();
 }

}