Stack #

public class StackAsArrayList {
    ArrayList<Integer> stack = new ArrayList<>();

    public boolean push(int value) {
        return stack.add(value);  // Adds an element to the end of the list
    }

    public int pop() {
        return stack.remove(stack.size() - 1);  // Removes the last element
    }

    public int peek() {
        return stack.get(stack.size() - 1);  // Returns the last element without removing
    }

    public int size() {
        return stack.size();  // Returns the size of the stack
    }
}

public class StackAsLinkedList {
    Node top;
    int size;

    public class Node {
        int value;
        Node next;

        public Node(int value) {
            this.value = value;
        }
    }

    public void push(int value) {
        Node newNode = new Node(value);  // Creates a new node
        newNode.next = top;  // Sets the next reference to the current top
        top = newNode;  // Updates the top to the new node
        size++;
    }

    public Node pop() {
        if (size == 0) return null;  // Stack underflow check
        Node popped = top;  // Get the top element
        top = top.next;  // Update top to the next element
        size--;
        return popped;
    }

    public Node peek() {
        return top;  // Returns the top node without removing
    }

    public int size() {
        return size;  // Returns the current size
    }
}

public class StackAsResizingArray {
    int[] stack = new int[1];  // Start with an initial capacity of 1
    int size = 0;

    public void push(int value) {
        if (isFull()) resize();  // Resize the array if it's full
        stack[size++] = value;  // Add the value and increment size
    }

    public int pop() {
        if (size == 0) throw new NoSuchElementException("Stack is empty");
        return stack[--size];  // Decrement size and return the popped element
    }

    public int peek() {
        if (size == 0) throw new NoSuchElementException("Stack is empty");
        return stack[size - 1];  // Return the top element without removing
    }

    public int size() {
        return size;  // Return the current size
    }

    private boolean isFull() {
        return size == stack.length;  // Check if the stack is full
    }

    private void resize() {
        stack = Arrays.copyOf(stack, stack.length * 2);  // Double the array size
    }
}