Find the Town Judge

Problem Statement - link #

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.

Examples #

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 <= 10^4
• trust[i].length == 2
• All the pairs of trust are unique.
• ai != bi
• 1 <= ai, bi <= n

Solutions #

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