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997. Find the Town Judge (Easy)

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:

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 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. 1 <= N <= 1000
2. trust.length <= 10000
3. trust[i] are all different
4. trust[i][0] != trust[i][1]
5. 1 <= trust[i][0], trust[i][1] <= N

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Solution 1.

// OJ: https://leetcode.com/problems/find-the-town-judge/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(N)
class Solution {
public:
    int findJudge(int N, vector<vector<int>>& trust) {
        vector<int> indegree(N + 1), outdegree(N + 1);
        for (auto &t : trust) {
            outdegree[t[0]]++;
            indegree[t[1]]++;
        }
        int judge = -1;
        for (int i = 1; i <= N; ++i) {
            if (indegree[i] != N - 1 || outdegree[i] != 0) continue;
            if (judge != -1) return false;
            judge = i;
        }
        return judge;
    }
};