# 1583. Count Unhappy Friends

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 `pairs[i] = [xi, yi]` denotes `xi` is paired with `yi` and `yi` is paired with `xi`.

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 = [, ], 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`
• `xi != yi`
• `0 <= xi, yi <= n - 1`
• Each person is contained in exactly one pair.

Medium

Normal

Bloomberg

### Solution(Chinese):

LEETCODE 1583. Count Unhappy Friends 解题思路分析