# 210. Course Schedule II

Medium
There are a total of `numCourses` courses you have to take, labeled from `0` to `numCourses - 1`. You are given an array `prerequisites` where `prerequisites[i] = [ai, bi]` indicates that you must take course `bi` first if you want to take course `ai`.
• For example, the pair `[0, 1]`, indicates that to take course `0` you have to first take course `1`.
Return the ordering of courses you should take to finish all courses. If there are many valid answers, return any of them. If it is impossible to finish all courses, return an empty array.
Example 1:
Input: numCourses = 2, prerequisites = [[1,0]]
Output: [0,1]
Explanation: There are a total of 2 courses to take. To take course 1 you should have finished course 0. So the correct course order is [0,1].
Example 2:
Input: numCourses = 4, prerequisites = [[1,0],[2,0],[3,1],[3,2]]
Output: [0,2,1,3]
Explanation: There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0.
So one correct course order is [0,1,2,3]. Another correct ordering is [0,2,1,3].
Example 3:
Input: numCourses = 1, prerequisites = []
Output: 
Constraints:
• `1 <= numCourses <= 2000`
• `0 <= prerequisites.length <= numCourses * (numCourses - 1)`
• `prerequisites[i].length == 2`
• `0 <= ai, bi < numCourses`
• `ai != bi`
• All the pairs `[ai, bi]` are distinct.

### 解題

Runtime: 7 ms, faster than 99.22%
Memory Usage: 6.1 MB, less than 96.50%
func findOrder(numCourses int, prerequisites [][]int) []int {
// Topological sort
edges := make([][]int, numCourses)
indegree := make([]int, numCourses ) //入邊, 入邊 = 0 = 沒有需要先修的課
for _, pre := range prerequisites {
edges[pre] = append(edges[pre], pre)
indegree[pre]++
}
queue, res := []int{}, []int{}
for i, v := range indegree { if v == 0 { queue = append(queue, i) } } // 先將沒有入邊的放到 queue
for len(queue) != 0 {
queue = queue[1:]