Recursive divide-and-conquer on string. In range [l, r), find lexicographically smallest character — its position determines output order.

Process right subrange before left subrange. Reason: after fixing smallest char at position k, right side (k+1..r) produces smaller prefixes than left side (l..k-1).

Base case: empty range returns. Mark visited characters; before each recursion call, print current state (visited chars shown at original positions).