1942 - The Number of the Smallest Unoccupied Chair.

Medium

There is a party where n friends numbered from 0 to n - 1 are attending. There is an infinite number of chairs in this party that are numbered from 0 to infinity. When a friend arrives at the party, they sit on the unoccupied chair with the smallest number.

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

When a friend leaves the party, their chair becomes unoccupied at the moment they leave. If another friend arrives at that same moment, they can sit in that chair.

You are given a 0-indexed 2D integer array times where <code>timesi = arrival_i, leaving_i</code>, indicating the arrival and leaving times of the <code>i^th</code> friend respectively, and an integer targetFriend. All arrival times are distinct.

Return the chair number that the friend numbered targetFriend will sit on.

Example 1:

Input: times = \[\[1,4],2,3,4,6], targetFriend = 1

Output: 1

Explanation:

• Friend 0 arrives at time 1 and sits on chair 0.

• Friend 1 arrives at time 2 and sits on chair 1.

• Friend 1 leaves at time 3 and chair 1 becomes empty.

• Friend 0 leaves at time 4 and chair 0 becomes empty.

• Friend 2 arrives at time 4 and sits on chair 0.

Since friend 1 sat on chair 1, we return 1.

Example 2:

Input: times = \[\[3,10],1,5,2,6], targetFriend = 0

Output: 2

Explanation:

• Friend 1 arrives at time 1 and sits on chair 0.

• Friend 2 arrives at time 2 and sits on chair 1.

• Friend 0 arrives at time 3 and sits on chair 2.

• Friend 1 leaves at time 5 and chair 0 becomes empty.

• Friend 2 leaves at time 6 and chair 1 becomes empty.

• Friend 0 leaves at time 10 and chair 2 becomes empty.

Since friend 0 sat on chair 2, we return 2.

Constraints:

• n == times.length

• <code>2 <= n <= 10⁴</code>

• times[i].length == 2

• <code>1 <= arrival_i< leaving_i<= 10⁵</code>

• 0 <= targetFriend <= n - 1

• Each <code>arrival_i</code> time is distinct.