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public class Stack<T> {
    // inner Node class
    private static class Node<T> {
        private T data;
        private Node<T> next;
        
        public Node(T data) {
            this.data = data;
        }
    }
    // inner Node class end
    
    private Node<T> top;
    
    public void push(T data) {
        Node<T> node = new Node<>(data);
        node.next = top;
        top = node;
    }
    
    public T pop() {
        if (top == null) throw new EmptyStackException();
        T val = top.data;
        top = top.next;
        return val;
    }
    
    public T peek() {
        if (top == null) throw new EmptyStackException();
        return top.data;
    }
    
    public boolean isEmpty() {
        return top == null;
    }
}