Problem

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:

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

Constraints:

• 1 <= N <= 1000
• 0 <= trust.length <= 10^4
• trust[i].length == 2
• trust[i] are all different
• trust[i][0] != trust[i][1]
• 1 <= trust[i][0], trust[i][1] <= N

Solution

The inputs are N as an integer and trust as a 2–D array.

To better understand the relationship between town judge and everybody, we can draw out the example:

Example 1:

1 has one-way connection to 2, and 2 doesn’t have connection with others. 2 doesn’t trust anyone, and everybody else trusts 2, so 2 is the town judge. It is a directed graph.

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

Output: 2

Example 2:

1 and 2 both have one-way connection to 3, and 3 doesn’t have outgoing pointer connected to others. Which makes 3 the town judge.

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

Output: 3

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