you're not querying in the correct order necessarily. A->B does not imply B->A.

vector<int> d = a;
sort(d.begin(), d.end());
d.resize(unique(d.begin(), d.end()) - d.begin());
for (int i = 0; i < n; ++i) {
  a[i] = lower_bound(d.begin(), d.end(), a[i]) - d.begin();
}
//original value of a[i] can be obtained through d[a[i]]