for the Problem 1849C

The SOLUTION I wrote, it is having some out of bounds error in the below code portion.

    s += (char)('0' ^ '1' ^ s.back());
    s = ("" + (char)('0' ^ '1' ^ s[0])) + s; // this line giving error 'out of bounds'
    n = s.size();

I am not able detect the exact reason.

Same purpose If I try to do like below, its not throwing any error

// working as expected
    if (s.size() > 0) s += (char)('0' ^ '1' ^ s.back());
    reverse(all(s));
    if (s.size() > 0) s += (char)('0' ^ '1' ^ s.back());
    reverse(all(s));
    n = s.size();