Codeforces 1375D Replace by MEX - Problem Statement

Given array $a$ of $n$ elements where $0 \le a_i \le n$, transform it into sorted order. Each operation: pick index $i$ and set $a_i = \text{MEX}(a)$.

Constraints: $n \le 1000$. Use at most $2n$ operations.

MEX is the minimum non-negative integer not in the array.