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 n1
.
All the friends are divided into pairs. The pairings are given in a list pairs
, where pairs[i] = [x_{i}, y_{i}]
denotes x_{i}
is paired with y_{i}
and y_{i}
is paired with x_{i}
.
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
prefersu
overy
, andu
prefersx
overv
.
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 containi
. All values in
preferences[i]
are unique. pairs.length == n/2
pairs[i].length == 2
x_{i} != y_{i}
0 <= x_{i}, y_{i} <= n  1
 Each person is contained in exactly one pair.

