Solution: The Number of the Smallest Unoccupied Chair

Statement

At a party, $n$ friends, numbered from $0$ to $n - 1$, arrive and leave at different times. There are infinitely many chairs, numbered $0$ onwards. Each arriving friend sits on the smallest available chair at that moment.

For example, if chairs $0$, $1$, and $5$ are occupied when a friend arrives, they will sit on chair number $2$.

When a friend leaves, their chair becomes immediately available. If another friend arrives simultaneously, they can take that chair.

You are given a $0$-indexed $2D$ list ...