Character Arithmetic Problem

Question

I was testing my C++ knowledge in Challenge mode when I came across this problem:
#include <iostream>
using namespace std;

char*f(char* const s, int n){
int i = 0;
n--;
while(i<n){
s[i]+=s[n];
s[n]= s[i] - s[n];
s[i] -=s[n];
i++; --n;
}
return s;
}
int main(){
char s[] = "desrever";
cout << f(s, 6+2);
return 0;
}
What's the trick here? It went by so fast I couldn't solve it in time, so there must be something REALLY obvious that I'm missing. Starting from the top, I need to add s[i]+=s[n], or d+=r. Was I just expected to know the numeric values of the letters or is there something else. Any help would be appreciated.

Answer 1

Thanks @-swim-, I would have never figured that out on my own. I still have a lot left to learn....