package StackArray;

public class Stack<Item> {

private Item[] stack;

private int numberOfItems;

public Stack(){

this.stack = ( Item[] ) new Object[1];

}

public void push(Item item){

if( numberOfItems == this.stack.length ){

resize(2*this.stack.length);

}

this.stack[numberOfItems++] = item;

}

public Item pop(){

Item itemToPop = this.stack[--numberOfItems];

if( numberOfItems > 0 && numberOfItems == this.stack.length/4 ){

resize(this.stack.length/2);

}

return itemToPop;

}

public boolean isEmpty(){

return this.numberOfItems == 0;

}

public int size(){

return this.numberOfItems;

}

// O(n)

private void resize(int capacity) {

Item[] stackCopy = ( Item[] ) new Object[capacity];

for(int i=0;i<numberOfItems;i++){

stackCopy[i]=this.stack[i];

}

this.stack = stackCopy;

}

}