// Stack implementation with arrays

public class CharStack {
    private char [] items;
    private int top;
	
    // a constructor, need for every class
    public CharStack() {
	items = new char [1000];
	top = -1;
    }
	
    public char pop() {
	if(top <= -1) {
	    System.out.println("Stack underflow");
	    System.exit(0);					
	}
	// if we got here, the stack is not empty
	top--;
	return items[top + 1]; 
    }
	
    public void push(char c) {
	if (top >= items.length - 1) {
	    System.out.println("Stack overflow");
	    System.exit(0);
	}
	// if we got here, the stack is not full
	top++;
	items[top]=c;
    }

    public boolean isEmpty() {
	return top == -1;	
    }
}