LeetCode-in-Java

1733. Minimum Number of People to Teach

Medium

On a social network consisting of m users and some friendships between users, two users can communicate with each other if they know a common language.

You are given an integer n, an array languages, and an array friendships where:

• There are n languages numbered 1 through n,
• languages[i] is the set of languages the ith user knows, and
• friendships[i] = [ui, vi] denotes a friendship between the users ui and vi.

You can choose one language and teach it to some users so that all friends can communicate with each other. Return the minimum number of users you need to teach.

Note that friendships are not transitive, meaning if x is a friend of y and y is a friend of z, this doesn’t guarantee that x is a friend of z.

Example 1:

Input: n = 2, languages = [[1],[2],[1,2]], friendships = [[1,2],[1,3],[2,3]]

Output: 1

Explanation: You can either teach user 1 the second language or user 2 the first language.

Example 2:

Input: n = 3, languages = [[2],[1,3],[1,2],[3]], friendships = [[1,4],[1,2],[3,4],[2,3]]

Output: 2

Explanation: Teach the third language to users 1 and 3, yielding two users to teach.

Constraints:

• 2 <= n <= 500
• languages.length == m
• 1 <= m <= 500
• 1 <= languages[i].length <= n
• 1 <= languages[i][j] <= n
• 1 <= ui < vi <= languages.length
• 1 <= friendships.length <= 500
• All tuples (ui, vi) are unique
• languages[i] contains only unique values

Solution

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class Solution {
    public int minimumTeachings(int n, int[][] languages, int[][] friendships) {
        int m = languages.length;
        boolean[][] speak = new boolean[m + 1][n + 1];
        boolean[][] teach = new boolean[m + 1][n + 1];
        for (int user = 0; user < m; user++) {
            int[] userLanguages = languages[user];
            for (int userLanguage : userLanguages) {
                speak[user + 1][userLanguage] = true;
            }
        }
        List<int[]> listToTeach = new ArrayList<>();
        for (int[] friend : friendships) {
            int userA = friend[0];
            int userB = friend[1];
            boolean hasCommonLanguage = false;
            for (int language = 1; language <= n; language++) {
                if (speak[userA][language] && speak[userB][language]) {
                    hasCommonLanguage = true;
                    break;
                }
            }
            if (!hasCommonLanguage) {
                for (int language = 1; language <= n; language++) {
                    if (!speak[userA][language]) {
                        teach[userA][language] = true;
                    }
                    if (!speak[userB][language]) {
                        teach[userB][language] = true;
                    }
                }
                listToTeach.add(friend);
            }
        }
        int minLanguage = Integer.MAX_VALUE;
        int languageToTeach = 0;
        for (int language = 1; language <= n; language++) {
            int count = 0;
            for (int user = 1; user <= m; user++) {
                if (teach[user][language]) {
                    count++;
                }
            }
            if (count < minLanguage) {
                minLanguage = count;
                languageToTeach = language;
            }
        }
        Set<Integer> setToTeach = new HashSet<>();
        for (int[] friend : listToTeach) {
            int userA = friend[0];
            int userB = friend[1];
            if (!speak[userA][languageToTeach]) {
                setToTeach.add(userA);
            }
            if (!speak[userB][languageToTeach]) {
                setToTeach.add(userB);
            }
        }
        return setToTeach.size();
    }
}

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