1583 - Count Unhappy Friends\.

Medium

You are given a list of preferences for n friends, where n is always even.

For each person i, preferences[i] contains a list of friends sorted in the order of preference. In other words, a friend earlier in the list is more preferred than a friend later in the list. Friends in each list are denoted by integers from 0 to n-1.

All the friends are divided into pairs. The pairings are given in a list pairs, where <code>pairsi = x_i, y_i</code> denotes <code>x_i</code> is paired with <code>y_i</code> and <code>y_i</code> is paired with <code>x_i</code>.

However, this pairing may cause some of the friends to be unhappy. A friend x is unhappy if x is paired with y and there exists a friend u who is paired with v but:

• x prefers u over y, and

• u prefers x over v.

Return the number of unhappy friends.

Example 1:

Input: n = 4, preferences = \[\[1, 2, 3], 3, 2, 0, 3, 1, 0, 1, 2, 0], pairs = \[\[0, 1], 2, 3]

Output: 2

Explanation:

Friend 1 is unhappy because:

• 1 is paired with 0 but prefers 3 over 0, and

• 3 prefers 1 over 2.

Friend 3 is unhappy because:

• 3 is paired with 2 but prefers 1 over 2, and

• 1 prefers 3 over 0.

Friends 0 and 2 are happy.

Example 2:

Input: n = 2, preferences = \[\[1], 0], pairs = \[\[1, 0]]

Output: 0

Explanation: Both friends 0 and 1 are happy.

Example 3:

Input: n = 4, preferences = \[\[1, 3, 2], 2, 3, 0, 1, 3, 0, 0, 2, 1], pairs = \[\[1, 3], 0, 2]

Output: 4

Constraints:

• 2 <= n <= 500

• n is even.

• preferences.length == n

• preferences[i].length == n - 1

• 0 <= preferences[i][j] <= n - 1

• preferences[i] does not contain i.

• All values in preferences[i] are unique.

• pairs.length == n/2

• pairs[i].length == 2

• <code>x_i != y_i</code>

• <code>0 <= x_i, y_i<= n - 1</code>

• Each person is contained in exactly one pair.