Tags: "leetcode", "graph", access_time 2-min read

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Find the Town Judge

Created: May 10, 2020 by [lek-tin]

Last updated: May 10, 2020

In a town, there are N people labelled from 1 to N. There is a rumor that one of these people is secretly the town judge.

If the town judge exists, then:

  • The town judge trusts nobody.
  • Everybody (except for the town judge) trusts the town judge.
  • There is exactly one person that satisfies properties 1 and 2.
  • You are given trust, an array of pairs trust[i] = [a, b] representing that the person labelled a trusts the person labelled b.

If the town judge exists and can be identified, return the label of the town judge. Otherwise, return -1.

Example 1:

Input: N = 2, trust = [[1,2]]
Output: 2

Example 2:

Input: N = 3, trust = [[1,3],[2,3]]
Output: 3

Example 3:

Input: N = 3, trust = [[1,3],[2,3],[3,1]]
Output: -1

Example 4:

Input: N = 3, trust = [[1,2],[2,3]]
Output: -1

Example 5:

Input: N = 4, trust = [[1,3],[1,4],[2,3],[2,4],[4,3]]
Output: 3

Note:

  • 1 <= N <= 1000
  • trust.length <= 10000
  • trust[i] are all different
  • trust[i][0] != trust[i][1]
  • 1 <= trust[i][0], trust[i][1] <= N

Solution (hashset)

Java

class Solution {
    public int findJudge(int N, int[][] trust) {
        HashMap<Integer, HashSet<Integer>> lookup = new HashMap<Integer, HashSet<Integer>>();
        for (int i = 0; i < trust.length; i++) {
            int from = trust[i][0];
            int to = trust[i][1];
            if (lookup.get(from) == null) {
                lookup.put(from, new HashSet<Integer>());
            }
            lookup.get(from).add(to);
        }

        int missingCount = 0;
        int judge = -1;
        for (int i = 1; i <= N; i++) {
            if (lookup.get(i) == null) {
                if (missingCount == 1) return -1;
                missingCount++;
                judge = i;
                continue;
            }
        }

        for (Map.Entry<Integer, HashSet<Integer>> entry: lookup.entrySet()) {
            HashSet<Integer> values = entry.getValue();
            if (!values.contains(judge)) return -1;
        }

        return judge;
    }
}

Solution (indegrees)

Java

public int findJudge(int N, int[][] trust) {
    // early stop
    if (trust.length < N - 1) {
        return -1;
    }

    int[] indegrees = new int[N + 1];
    int[] outdegrees = new int[N + 1];

    for (int[] relation : trust) {
        int from = relation[0];
        int to = relation[1];
        outdegrees[from]++;
        indegrees[to]++;
    }

    for (int i = 1; i <= N; i++) {
        if (indegrees[i] == N - 1 && outdegrees[i] == 0) {
            return i;
        }
    }
    return -1;
}