1942. The Number of the Smallest Unoccupied Chair

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, thei