997 Find the Town Judge.py

#!/usr/bin/python3
"""
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:

The town judge trusts nobody.
Everybody (except for the town judge) trusts the town judge.
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 <= N <= 1000
trust.length <= 10000
trust[i] are all different
trust[i][0] != trust[i][1]
1 <= trust[i][0], trust[i][1] <= N
"""
from typing import List
from collections import defaultdict


class Solution:
    def findJudge(self, N: int, trust: List[List[int]]) -> int:
        """
        like the find the celebrity
        """
        ingress = defaultdict(set)
        egress =defaultdict(set)
        for p, q in trust:
            egress[p].add(q)
            ingress[q].add(p)
        for i in range(1, N+1):
            if len(egress[i]) == 0 and len(ingress[i]) == N - 1:
                return i
        return -1