Find the Town Judge

Description

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(javascript)

/**
 * @param {number} N
 * @param {number[][]} trust
 * @return {number}
 */
const findJudge = function (N, trust) {
  const inDegrees = new Array(N + 1).fill(0)
  const outDegrees = new Array(N + 1).fill(0)
  trust.forEach(([a, b]) => {
    outDegrees[a] = (outDegrees[a] || 0) + 1
    inDegrees[b] = (inDegrees[b] || 0) + 1
  })
  for (let i = 1; i <= N; i++) {
    if (outDegrees[i] === 0 && inDegrees[i] === N - 1) {
      return i
    }
  }
  return -1
}