997. Find the Town Judge

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1039. Find the Town Judge

Description

Difficulty: Easy

Related Topics: Array, Hash Table, Graph

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

If the town judge exists, then:

  1. The town judge trusts nobody.
  2. Everybody (except for the town judge) trusts the town judge.
  3. There is exactly one person that satisfies properties 1 and 2.

You are given an array trust where trust[i] = [ai, bi] representing that the person labeled ai trusts the person labeled bi.

Return the label of the town judge if the town judge exists and can be identified, or return -1 otherwise.

Example 1:

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

Example 2:

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

Example 3:

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

Constraints:

  • 1 <= n <= 1000
  • 0 <= trust.length <= 104
  • trust[i].length == 2
  • All the pairs of trust are unique.
  • ai != bi
  • 1 <= ai, bi <= n

Hints/Notes

  • if the number of pairs is less than n - 1, then we cannot identify the judge
  • use in/out vector to mark the trust relationship

Solution

Language: C++

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class Solution {
public:
    int findJudge(int n, vector<vector<int>>& trusts) {
        int res = -1;
        if (trusts.size() < n - 1) {
            return res;
        }
        vector<int> in_trust(n + 1, 0);
        vector<int> out_trust(n + 1, -1);
        for (vector<int> t : trusts) {
            in_trust[t[1]]++;
            out_trust[t[0]] = t[1];
        }
        for (int i = 1; i <= n; i++) {
            if (in_trust[i] == n - 1 && out_trust[i] == -1) {
                return i;
            }
        }
        return -1;
    }
};