Leetcode #210: Course Schedule II

func findOrder(_ numCourses: Int, _ prerequisites: [[Int]]) -> [Int] {
    var inDegrees = Array(repeating: 0, count: numCourses)
    var dict = [Int: [Int]]()
    for prerequisite in prerequisites {
        inDegrees[prerequisite[0]] += 1
        if let list = dict[prerequisite[1]] {
            dict[prerequisite[1]] = list + [prerequisite[0]]
        } else {
            dict[prerequisite[1]] = [prerequisite[0]]
        }
    }
    var queue = [Int]()
    var answer = [Int]()
    for (index, inDegree) in inDegrees.enumerated() {
        if inDegree == 0 {
            queue.append(index)
        }
    }
    while !queue.isEmpty {
        let currentCourse = queue.removeFirst()
        answer.append(currentCourse)
        if let list = dict[currentCourse] {
            for course in list {
                inDegrees[course] -= 1
                if inDegrees[course] == 0 {
                    queue.append(course)
                }
            }
        }
    }
    return answer.count == numCourses ? answer : []
}