Find the Town Judge

Description

n 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. If a trust relationship does not exist in trust array, then such a trust relationship does not exist.

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

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

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

Code

Time Complexity: $O(n)$, Space Complexity: $O(n)$

class Solution {
public:
    int findJudge(int n, vector<vector<int>>& trust) {
        vector<int> inDegree(n, 0);
        vector<int> outDegree(n, 0);
        for(auto t: trust) {
            inDegree[t[1] - 1]++;
            outDegree[t[0] - 1]++;
        }
 
        for(int i = 0; i < n; i++) {
            if(outDegree[i] == 0) {
                if(inDegree[i] == n - 1) return i + 1;
            }
        }
 
        return -1;
    }
};

Source

• Find the Town Judge - LeetCode