#include <stdio.h>
#include <iostream>
using namespace std;
struct El {
    int data;
    El *next;
    El(int x) {
        data = x;
        next = NULL;
    }
};
El *top = NULL;
void push(int x) {
    El *t = new El(x);
    if (top == NULL) {
        top = t;
        return;
    }
    t->next = top;
    top = t;
}
int pop() {
    if (top == NULL) return 0;
    int res = top->data;
    top = top->next;
    return res;
}
int back() {
    if (top != NULL) return top->data;
}
int size() {
    if (top == NULL) return 0;
    El *t = top;
    int res = 0;
    while (t != NULL) {
        t = t->next;
        res++;
    }
    return res;
}
void cleanStack() {
 if (top != NULL) {
        pop();
        cleanStack();
    }
}