LeetCode 1951: All the Pairs With the Maximum Number of Common Followers Solution

import java.util.*;

class Solution {
    public List<List<Integer>> mostCommonFollowersPairs(int[][] followers) {
        Map<List<Integer>, Integer> pairCount = new HashMap<>();
        int maxCount = 0;

        for (int i = 0; i < followers.length; i++) {
            for (int j = i + 1; j < followers.length; j++) {
                List<Integer> pair = Arrays.asList(followers[i][0], followers[j][0]);
                int count = calculateCommonFollowers(followers[i], followers[j]);
                pairCount.put(pair, pairCount.getOrDefault(pair, 0) + count);
                maxCount = Math.max(maxCount, pairCount.get(pair));
            }
        }

        List<List<Integer>> result = new ArrayList<>();
        for (Map.Entry<List<Integer>, Integer> entry : pairCount.entrySet()) {
            if (entry.getValue() == maxCount) {
                result.add(entry.getKey());
            }
        }

        return result;
    }

    private int calculateCommonFollowers(int[] user1, int[] user2) {
        Set<Integer> followers1 = new HashSet<>();
        Set<Integer> followers2 = new HashSet<>();
        for (int i = 1; i < user1.length; i++) {
            followers1.add(user1[i]);
        }
        for (int i = 1; i < user2.length; i++) {
            followers2.add(user2[i]);
        }
        followers1.retainAll(followers2);
        return followers1.size();
    }
}