207. Course Schedule

       func canFinish(_ n: Int, _ prerequisites: [[Int]]) -> Bool {
        var adjentMap = [Int: Set<Int>]()
        var indegree = [Int](repeating: 0 , count: n)

        for prerequisite in prerequisites{
            let first = prerequisite[1]
            let second = prerequisite[0]
            indegree[second] += 1
            var newSet:Set<Int>
            if let list = adjentMap[first]{
                newSet = list
            }else {
                newSet = Set<Int>()
            }
            newSet.insert(second)
            adjentMap[first] = newSet
        }
        var queue = [Int]()
        for i in 0 ..< n  {
            if indegree[i] == 0{
                queue.append(i)
            }
        }
        var count = 0
        while !queue.isEmpty {
            count += 1
            let cur = queue.removeFirst()
            if let courseSet = adjentMap[cur] {
                for adj in courseSet {
                    indegree[adj] -= 1
                    if indegree[adj] == 0{
                        queue.append(adj)
                    }
                }
            }
        }

        return count == n
    }