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类型：qbank
You are given an array prerequisites where prerequisites[i] = [a, b] indicates that you must take course b first if you want to take course a.
Examples
Example 1:
Input: numCourses = 2, prerequisites = [[0,1]]
Output: true
Explanation:
First take course 1 (no prerequisites) and then take course 0.
Example 2:
Input: numCourses = 2, prerequisites = [[0,1],[1,0]]
Output: false
Explanation:
In order to take course 1 you must take course 0, and to take course 0 you must take course 1. So it is impossible.
Constraints
1 <= numCourses <= 1000
0 <= prerequisites.length <= 1000
prerequisites[i].length == 2
0 <= a[i], b[i] < numCourses
All prerequisite pairs are unique.