Count the people behind merged user accounts

A sign-up system creates duplicate accounts when one person registers twice. Support staff record merges: each one says that two accounts belong to the same person.

Write count_people(n, merges) that returns how many different people the accounts belong to.

Input and output:

• n: the number of accounts, from 1 to 100,000. Accounts are numbered 0 to n - 1.
• merges: a list of up to 100,000 pairs [a, b], where a and b are account numbers.
• Returns an integer.

Rules:

• Each account starts as its own person.
• Two accounts belong to the same person if a chain of merges connects them, in any order and in either direction.
• A merge can repeat an earlier one or name the same account twice. Neither changes the count.