LeetCode 997. Find the Town Judge

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题目

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:

  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 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:

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Input: N = 2, trust = [[1,2]]
Output: 2

Example 2:

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Input: N = 3, trust = [[1,3],[2,3]]
Output: 3

Example 3:

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Input: N = 3, trust = [[1,3],[2,3],[3,1]]
Output: -1

Example 4:

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Input: N = 3, trust = [[1,2],[2,3]]
Output: -1

Example 5:

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Input: N = 4, trust = [[1,3],[1,4],[2,3],[2,4],[4,3]]
Output: 3

Note:

  1. 1 <= N <= 1000
  2. trust.length <= 10000
  3. trust[i] are all different
  4. trust[i][0] != trust[i][1]
  5. 1 <= trust[i][0], trust[i][1] <= N

思路

Easy题打卡，图的入度和出度。
法官就是入度为N-1，出度为0的节点。

代码

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class Solution(object):
    def findJudge(self, N, trust):
        """
        :type N: int
        :type trust: List[List[int]]
        :rtype: int
        """
        in_degree = [0] * (N+1)
        out_degree = [0] * (N+1)
        for i in trust:
            out_degree[i[0]] += 1
            in_degree[i[1]] += 1
        for i in range(1, N+1):
            if in_degree[i] == N-1 and out_degree[i] == 0:
                return i
        return -1