# Stack

A Last-in-first-out (LIFO) Data Structure

## Implementation - Stack

A **dynamic array** is sufficient to implement a **stack** structure.

Sample Implementation (Using Java **AraryList** structure)

```java
// "static void main" must be defined in a public class.
class MyStack {
    private List<Integer> data;               // store elements
    public MyStack() {
        data = new ArrayList<>();
    }
    /** Insert an element into the stack. */
    public void push(int x) {
        data.add(x);
    }
    /** Checks whether the queue is empty or not. */
    public boolean isEmpty() {
        return data.isEmpty();
    }
    /** Get the top item from the queue. */
    public int top() {
        return data.get(data.size() - 1);
    }
    /** Delete an element from the queue. Return true if the operation is successful. */
    public boolean pop() {
        if (isEmpty()) {
            return false;
        }
        data.remove(data.size() - 1);
        return true;
    }
};

public class Main {
    public static void main(String[] args) {
        MyStack s = new MyStack();
        s.push(1);
        s.push(2);
        s.push(3);
        for (int i = 0; i < 4; ++i) {
            if (!s.isEmpty()) {
                System.out.println(s.top());
            }
            System.out.println(s.pop());
        }
    }
}
```

## Stack Usage

Java has a class called java.util.Stack:

`pop()`, `push()`

```java
// "static void main" must be defined in a public class.
public class Main {
    public static void main(String[] args) {
        // 1. Initialize a stack.
        Stack<Integer> s = new Stack<>();
        // 2. Push new element.
        s.push(5);
        s.push(13);
        s.push(8);
        s.push(6);
        // 3. Check if stack is empty.
        if (s.empty() == true) {
            System.out.println("Stack is empty!");
            return;
        }
        // 4. Pop an element.
        s.pop();
        // 5. Get the top element.
        System.out.println("The top element is: " + s.peek());
        // 6. Get the size of the stack.
        System.out.println("The size is: " + s.size());
    }
}
```

According to [Java doc](https://docs.oracle.com/javase/8/docs/api/java/util/Stack.html)

> A more complete and consistent set of LIFO stack operations is provided by the Deque interface and its implementations, which should be used in preference to this class.

Use

```
Deque<Integer> stack = new ArrayDeque<>();
```

Maybe better


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