public class Stack {
	private int size=0;
	private String[] q=new String[0];
	/** Adds a value to the top of the stack.*/
	public void push(String value){
		size++;
		String[] m=new String[size];
		m[0]=value;
		for(int x=1; x<size; x++){
			m[x]=q[x-1];
		}
		q=m;
	}

	/** Removes the topmost string. If the stack is empty, returns null. */
	public String pop() {
		if(size==0)
			return null;
		else
			size--;
		String[] m=new String[size];
		String w = q[size];
		for(int x=0; x<size; x++){
			m[x]=q[x];
		}
		q=m;
		return w;
	}

	/** Returns the topmost string but does not remove it. If the stack is empty, returns null. */
	public String peek() {
		if(isEmpty())
			return null;
		else
			return q[size-1];
	}

	/** Returns true iff the stack is empty */
	public boolean isEmpty() {
		return (size==0);
	}

	/** Returns the number of items in the stack. */
	public int length() {
		return size;
	}

	/** Returns a string representation of the stack. Each string is separated by a newline. Returns an empty string if the stack is empty. */
	public String toString() {
		String s="";
		if(isEmpty())
			return s;
		for(int x=size-1; x>0; x--){
			s+=q[x]+"\n";
		}
		s+=q[0];
		return s;
	}
}