???为啥注释掉的不能AC

#include <iostream>
#include <iomanip>
#include <vector>
#include <string>
#include <cstring>
#include <cmath>
using namespace std;

int main() {
    int n;
    cin >> n;
    string str;
    cin >> str;
    int size = str.size();
    for (int i = 0; i < size; i++) {
       // char j = str[i];
       //j += n;
       //str[i]='a' +(j-'a')%26;
        str[i] = 'a' + (str[i] + n - 'a') % 26;

    }
    cout << str;
    return 0;
}