997.py

'''
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
'''

class Solution(object):
    def findJudge(self, N, trust):
        """
        :type N: int
        :type trust: List[List[int]]
        :rtype: int
        """
        if not trust:
            return 1
        mapping = {}
        unique = set()
        for truste_list in trust:
            unique.add(truste_list[0])
            if truste_list[1] in mapping:
                mapping[truste_list[1]] += 1
            else:
                mapping[truste_list[1]] = 1
            
        unique_set = len(unique)
        for key, value in mapping.items():
            if value == unique_set:
                return key
        return -1