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: [0]
Constraints:
1 <= numCourses <= 20000 <= prerequisites.length <= numCourses * (numCourses - 1)prerequisites[i].length == 20 <= ai, bi < numCoursesai != bi- All the pairs
[ai, bi]are distinct.
class Solution:
def findOrder(self, numCourses: int, prerequisites: List[List[int]]) -> List[int]:
edges = defaultdict(list)
indegree = [0] * numCourses
for a, b in prerequisites:
edges[b].append(a)
indegree[a] += 1
q = deque()
for i in range(numCourses):
if indegree[i] == 0:
q.append(i)
ans = []
while q:
b = q.popleft()
ans.append(b)
for a in edges[b]:
indegree[a] -= 1
if indegree[a] == 0:
q.append(a)
return ans if len(ans) == numCourses else []class Solution {
public int[] findOrder(int numCourses, int[][] prerequisites) {
List<Integer>[] edges = new List[numCourses];
for (int i = 0; i < numCourses; ++i) {
edges[i] = new ArrayList<>();
}
int[] indegree = new int[numCourses];
for (int[] p : prerequisites) {
int a = p[0], b = p[1];
edges[b].add(a);
++indegree[a];
}
Queue<Integer> q = new LinkedList<>();
for (int i = 0; i < numCourses; ++i) {
if (indegree[i] == 0) {
q.offer(i);
}
}
int[] ans = new int[numCourses];
int n = 0;
while (!q.isEmpty()) {
int b = q.poll();
ans[n++] = b;
for (int a : edges[b]) {
if (--indegree[a] == 0) {
q.offer(a);
}
}
}
return n == numCourses ? ans : new int[0];
}
}function findOrder(numCourses: number, prerequisites: number[][]): number[] {
let edges = Array.from({ length: numCourses }, () => ([]));
let indeg = new Array(numCourses).fill(0);
for (let [b, a] of prerequisites) {
edges[a].push(b);
indeg[b] += 1;
}
let queue = [];
for (let i = 0; i < numCourses; i++) {
if (!indeg[i]) {
queue.push(i);
}
}
let ans = [];
while (queue.length) {
const u = queue.shift();
ans.push(u);
for (let v of edges[u]) {
indeg[v] -= 1;
if (!indeg[v]) {
queue.push(v);
}
}
}
return ans.length == numCourses ? ans : [];
};class Solution {
public:
vector<int> findOrder(int numCourses, vector<vector<int>>& prerequisites) {
vector<vector<int>> edges(numCourses);
vector<int> indegree(numCourses);
for (auto& p : prerequisites)
{
int a = p[0], b = p[1];
edges[b].push_back(a);
++indegree[a];
}
queue<int> q;
for (int i = 0; i < numCourses; ++i)
if (indegree[i] == 0)
q.push(i);
vector<int> ans;
while (!q.empty())
{
int b = q.front();
q.pop();
ans.push_back(b);
for (int a : edges[b])
if (--indegree[a] == 0)
q.push(a);
}
return ans.size() == numCourses ? ans : vector<int>();
}
};func findOrder(numCourses int, prerequisites [][]int) []int {
edges := make([][]int, numCourses)
indegree := make([]int, numCourses)
for _, p := range prerequisites {
a, b := p[0], p[1]
edges[b] = append(edges[b], a)
indegree[a]++
}
var q []int
for i := 0; i < numCourses; i++ {
if indegree[i] == 0 {
q = append(q, i)
}
}
var ans []int
for len(q) > 0 {
b := q[0]
q = q[1:]
ans = append(ans, b)
for _, a := range edges[b] {
indegree[a]--
if indegree[a] == 0 {
q = append(q, a)
}
}
}
if len(ans) == numCourses {
return ans
}
return []int{}
}public class Solution {
public int[] FindOrder(int numCourses, int[][] prerequisites) {
var edges = new List<int>[numCourses];
for (int i = 0; i < numCourses; ++i)
{
edges[i] = new List<int>();
}
var indegree = new int[numCourses];
for (int i = 0; i < prerequisites.Length; ++i)
{
int a = prerequisites[i][0];
int b = prerequisites[i][1];
edges[b].Add(a);
++indegree[a];
}
var q = new Queue<int>();
for (int i = 0; i < numCourses; ++i)
{
if (indegree[i] == 0) q.Enqueue(i);
}
var ans = new int[numCourses];
var n = 0;
while (q.Count > 0)
{
int b = q.Dequeue();
ans[n++] = b;
foreach (int a in edges[b])
{
if (--indegree[a] == 0) q.Enqueue(a);
}
}
return n == numCourses ? ans : new int[0];
}
}