QAQ.cpp

/* correct */
for (int i = 1; i < n; ++i) {
    if (ants[i].pos == ants[i - 1].pos) {
        ants[i].direction = 'T';
        ants[i - 1].direction = 'T';
    }
}
 
/* wrong */
for (auto it = ants.begin() + 1; it != ants.begin() + n; ++it) {
    if (it->pos == (it - 1)->pos) {
        it->direction = 'T';
        (it - 1)->direction = 'T';
    }
}