LeetCode in Kotlin

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 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.

Solution

@Suppress("UNUSED_PARAMETER")
class Solution {
    fun unhappyFriends(n: Int, preferences: Array<IntArray>, pairs: Array<IntArray>): Int {
        var unhappyFriends = 0
        val assignedPair: MutableMap<Int, Int> = HashMap()
        for (pair in pairs) {
            assignedPair[pair[0]] = pair[1]
            assignedPair[pair[1]] = pair[0]
        }
        for (pair in pairs) {
            if (isUnHappy(pair[1], pair[0], preferences, assignedPair)) {
                unhappyFriends++
            }
            if (isUnHappy(pair[0], pair[1], preferences, assignedPair)) {
                unhappyFriends++
            }
        }
        return unhappyFriends
    }

    private fun isUnHappy(
        self: Int,
        assignedFriend: Int,
        preferences: Array<IntArray>,
        assignedPairs: Map<Int, Int>,
    ): Boolean {
        val preference = preferences[self]
        val assignedFriendPreferenceIndex = findIndex(preference, assignedFriend)
        for (i in 0..assignedFriendPreferenceIndex) {
            val preferredFriend = preference[i]
            val preferredFriendAssignedFriend = assignedPairs[preferredFriend]!!
            if (preferredFriendAssignedFriend == self) {
                return false
            }
            val candidateAssignedFriendIndex = findIndex(preferences[preferredFriend], preferredFriendAssignedFriend)
            if (isPreferred(self, preferences[preferredFriend], candidateAssignedFriendIndex)) {
                return true
            }
        }
        return false
    }

    private fun isPreferred(self: Int, preference: IntArray, boundary: Int): Boolean {
        for (i in 0..boundary) {
            if (self == preference[i]) {
                return true
            }
        }
        return false
    }

    private fun findIndex(preference: IntArray, assignedFriend: Int): Int {
        for (i in preference.indices) {
            if (preference[i] == assignedFriend) {
                return i
            }
        }
        return 0
    }
}

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