## 210. Course Schedule II

There are a total of `n` courses you have to take labelled from `0` to `n - 1`.

Some courses may have `prerequisites`, for example, if `prerequisites[i] = [ai, bi]` this means you must take the course `bi` before the course `ai`.

Given the total number of courses `numCourses` and a list of the `prerequisite` pairs, 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.

## Rust Solution

``````struct Solution;

use std::collections::VecDeque;

impl Solution {
fn find_order(num_courses: i32, prerequisites: Vec<Vec<i32>>) -> Vec<i32> {
let mut res = vec![];
let n = num_courses as usize;
let mut edges: Vec<Vec<usize>> = vec![vec![]; n];
let mut indegrees: Vec<usize> = vec![0; n];
let mut queue: VecDeque<usize> = VecDeque::new();
for e in prerequisites {
let u = e as usize;
let v = e as usize;
edges[u].push(v);
indegrees[v] += 1;
}
for u in 0..n {
if indegrees[u] == 0 {
queue.push_back(u);
}
}
while let Some(u) = queue.pop_front() {
res.push(u);
for &v in &edges[u] {
indegrees[v] -= 1;
if indegrees[v] == 0 {
queue.push_back(v);
}
}
}
if res.len() == n {
res.into_iter().map(|v| v as i32).collect()
} else {
vec![]
}
}
}

#[test]
fn test() {
let num_courses = 2;
let prerequisites: Vec<Vec<i32>> = vec_vec_i32![[1, 0]];
let res = vec![0, 1];
assert_eq!(Solution::find_order(num_courses, prerequisites), res);
let num_courses = 4;
let prerequisites: Vec<Vec<i32>> = vec_vec_i32![[1, 0], [2, 0], [3, 1], [3, 2]];
let res = vec![0, 1, 2, 3];
assert_eq!(Solution::find_order(num_courses, prerequisites), res);
}
``````

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