997. Find the Town Judge

1. Question

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] = [a_i, b_i] representing that the person labeled a_i trusts the person labeled b_i.

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

2. 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

3. Constraints

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

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/find-the-town-judge 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

理解成有向图，则法官的入度为n-1，出度为0.

class Solution {
  public int findJudge(int n, int[][] trust) {
    int[] in = new int[n + 1];
    int[] out = new int[n + 1];

    for(int[] arr : trust) {
      out[arr[0]]++;
      in[arr[1]]++;
    }



    for(int i = 1; i <= n; i++) {
      if(in[i] == n - 1 && out[i] == 0) {
        return i;
      }
    }
    return -1;
  }
}