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 times, where times[i] $= [ arrival_i, leaving_i]$ represents the arrival and departure times of the $i_{th}$ friend. All arrival times are unique.

Given an integer target_friend, return the chair number that target_friend will sit on.

Constraints:

• $n ==$ times.length

• times[i].length $== 2$

• $1\leq$ arrivali < leavingi $\leq 10^5$

• $0 \leq$ target_friend $\leq n - 1$

• Each arrivali time is unique.

Examples