Implement Queue/Stack

Queue: First-in-first-out

// Initialize a queue.
Queue<Integer> q = new LinkedList();

// Get the first element - return null if queue is empty.
q.peek();

// Push new element.
q.offer(5);

// Pop an element.
q.poll();

Stack: Last-in-fist-out

// Initialize a stack.
Stack<Integer> s = new Stack<>();

// Push new element.
s.push(5);

// Check if stack is empty.
if (s.empty() == true) {
    System.out.println("Stack is empty!");
    return;
}

// Pop an element.
s.pop();

// Get the top element.
s.peek();

// Get the size of the stack.
s.size();