207\. Course Schedule

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 <code>prerequisites[i] = [a<sub>i</sub>, b<sub>i</sub>]</code> indicates that you **must** take course <code>b<sub>i</sub></code> first if you want to take course <code>a<sub>i</sub></code>.

*   For example, the pair `[0, 1]`, indicates that to take course `0` you have to first take course `1`.

Return `true` if you can finish all courses. Otherwise, return `false`.

**Example 1:**

**Input:** numCourses = 2, prerequisites = [[1,0]]

**Output:** true

**Explanation:** There are a total of 2 courses to take. To take course 1 you should have finished course 0. So it is possible. 

**Example 2:**

**Input:** numCourses = 2, prerequisites = [[1,0],[0,1]]

**Output:** false

**Explanation:** There are a total of 2 courses to take. To take course 1 you should have finished course 0, and to take course 0 you should also have finished course 1. So it is impossible. 

**Constraints:**

*   <code>1 <= numCourses <= 10<sup>5</sup></code>
*   `0 <= prerequisites.length <= 5000`
*   `prerequisites[i].length == 2`
*   <code>0 <= a<sub>i</sub>, b<sub>i</sub> < numCourses</code>
*   All the pairs prerequisites[i] are **unique**.