[Swift]LeetCode1136. 平行课程 | Parallel Courses

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There are N courses, labelled from 1 to N.

We are given relations[i] = [X, Y], representing a prerequisite relationship between course X and course Y: course X has to be studied before course Y.

In one semester you can study any number of courses as long as you have studied all the prerequisites for the course you are studying.

Return the minimum number of semesters needed to study all courses.  If there is no way to study all the courses, return -1. 

Example 1:

Input: N = 3, relations = [[1,3],[2,3]]
Output: 2
Explanation: 
In the first semester, courses 1 and 2 are studied. In the second semester, course 3 is studied.

Example 2:

Input: N = 3, relations = [[1,2],[2,3],[3,1]]
Output: -1
Explanation: 
No course can be studied because they depend on each other. 

Note:

1. 1 <= N <= 5000
2. 1 <= relations.length <= 5000
3. relations[i][0] != relations[i][1]
4. There are no repeated relations in the input.

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有n个课程，从1到n标记。我们得到关系[i]=[x，y]，表示课程x和课程y之间的先决条件关系：课程x必须在课程y之前学习。在一个学期内，你可以学习任意数量的课程，只要你已经学习了你所学课程的所有先决条件。返回学习所有课程所需的最少学期数。如果没有办法学习所有课程，返回-1。

例1：

输入：n=3，relations=[[1,3]，[2,3]]

输出：2

说明：

第一学期，学习课程1和2。第二学期，学习第三门课程。

例2：

输入：n=3，relations=[[1,2]，[2,3]，[3,1]]

输出：-1

说明：

任何课程都不能学习，因为它们相互依赖。

提示：

1. 1 <= N <= 5000
2. 1 <= relations.length <= 5000
3. relations[i][0] != relations[i][1]
4. 输入中没有重复的关系