You are given an integer n, representing the number of friends labeled from $0$ to $n - 1$. You are also given an array preferences, where preferences[i] is a list of all other friends sorted in order of preference, from most preferred to least preferred.

The friends are divided into pairs represented by a $2 ext{D}$ array pairs, where pairs[i] = [x_i, y_i] indicates that friend x_i is paired with friend y_i. Each friend belongs to exactly one pair.

A friend x is unhappy if x is paired with y, and there exists another friend u who is paired with v such that both of the following conditions hold:

• x prefers u over y

• u prefers x over v

Return the total number of unhappy friends.

Constraints:

• $2 \leq$ n $\leq 500$

• n is even.

• preferences.length $= n$

• preferences[i].length $= n - 1$

• $0 \leq$ preferences[i][j] $\leq n - 1$

• preferences[i] does not contain i.

• All values in preferences[i] are unique.

• pairs.length $= n / 2$

• pairs[i].length $= 2$

• x_i $\neq$ y_i

• $0 \leq$ x_i, y_i $\leq n - 1$

• Each person is contained in exactly one pair.

Now, let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Implement your solution in the following coding playground.