public abstract class Lists {
    abstract void push(int i);
    abstract int pop();
}

class ArrayStack extends Lists {
    int[] is = new int[7];
    int count = 0;

    void push(int i) {
	is[count] = i;
	count++;
    }

    int pop() {
	--count;
	return is[count];
    }

    int size() {
	return count;
    }
}

class LinkedStack extends Lists {
    Node first;
    int count = 0;

    void push(int i) {
	first = new Node(i, first);
	count++;
    }

    int pop() {
	int i = first.i;
	first = first.n;
	count--;
	return i;
    }

    int size() {
	return count;
    }
}

class ArrayList extends Lists {
    int[] is = new int[7];
    int first = 0, last = 0;

    void push(int i) {
	is[first] = i;
	first++;
    }

    int pop() {
	int i = is[last];
	last++;
	return i;
    }

    int size() {
	return first - last;
    }
}

class LinkedList extends Lists {
    Node head = new Node(0), last = head;
    int count = 0;

    void push(int i) {
	Node n = new Node(i);
	last.n = n;
	last = n;
	count++;
    }

    int pop() {
	int i = head.n.i;
	head.n = head.n.n;
	count--;
	return i;
    }
}

class Node {
    Node n;
    int i;

    Node(int i) {this.i = i;}
    NOde(int i, Node n) {this(i); this.n = n;}
}