# ---
# title: 1583. Count Unhappy Friends
# id: problem1583
# author: Tian Jun
# date: 2020-10-31
# difficulty: Medium
# categories: Array
# link: <https://leetcode.com/problems/count-unhappy-friends/description/>
# hidden: true
# ---
# 
# 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 = [[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`
#   * `xi != yi`
#   * `0 <= xi, yi <= n - 1`
#   * Each person is contained in **exactly one** pair.
# 
# 
## @lc code=start
using LeetCode

## add your code here:
## @lc code=end
