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.

class Solution {
    bool helper(int me, int matched, int target, vector<int>& A) {
        for(auto& a : A) {
            if(a == matched) return true;
            if(a == target) return false;
        }
        return false;
    }
public:
    int unhappyFriends(int n, vector<vector<int>>& A, vector<vector<int>>& B) {
        int res = 0;
        vector<int> match(n);
        for(auto& b : B) {
            match[b[0]] = b[1];
            match[b[1]] = b[0];
        }
        for(int i = 0; i < n; i++) {
            bool happy = true;
            for(auto& pref : A[i]) {
                if(pref == match[i]) break;
                if(!happy) break;
                happy = helper(pref, match[pref], i, A[pref]);
            }
            res += !happy;
        }
        return res;
    }
};