I have written a recursive code to calculate sum of first n numbers, as I am not returning s, but code is working. Why?

I have written a recursive code in CPP to calculate sum of first n numbers, as I am not returning s, but code is working fine. Is ‘s’ variable is shared with all functions.

#include<iostream>
using namespace std;

int sum(int n){
    int s = 0;
    if(n == 0)  return 0;
    s = n + sum(n - 1);
}

int main(){
    int n; 
    cin >> n;
    int s = sum(n);
    cout << s;
    return 0;
}

>Solution :

You should return an integer from sum if n is not equal to 0.

#include<iostream>
using namespace std;

int sum(int n){
    if(n == 0)  return 0;
    return n + sum(n - 1);
}

int main(){
    int n; 
    cin >> n;
    int s = sum(n);
    cout << s;
    return 0;
}