60. Course Schedule II

class Solution:
    def findOrder(self, numCourses: int, prerequisites: List[List[int]]) -> List[int]:
        adj = [[] for _ in range(numCourses)]
        indeg = [0] * numCourses
        for a, b in prerequisites:
            adj[b].append(a)
            indeg[a] += 1
        q = deque(i for i in range(numCourses) if indeg[i] == 0)
        order = []
        while q:
            u = q.popleft()
            order.append(u)
            for v in adj[u]:
                indeg[v] -= 1
                if indeg[v] == 0:
                    q.append(v)
        return order if len(order) == numCourses else []

class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        List<List<Integer>> adj = new ArrayList<>();
        for (int i = 0; i < numCourses; i++) adj.add(new ArrayList<>());
        int[] indeg = new int[numCourses], order = new int[numCourses];
        for (int[] p : prerequisites) { adj.get(p[1]).add(p[0]); indeg[p[0]]++; }
        ArrayDeque<Integer> q = new ArrayDeque<>();
        for (int i = 0; i < numCourses; i++) if (indeg[i] == 0) q.add(i);
        int k = 0;
        while (!q.isEmpty()) {
            int u = q.poll();
            order[k++] = u;
            for (int v : adj.get(u)) if (--indeg[v] == 0) q.add(v);
        }
        return k == numCourses ? order : new int[0];
    }
}

class Solution {
public:
    vector<int> findOrder(int numCourses, vector<vector<int>>& prerequisites) {
        vector<vector<int>> adj(numCourses);
        vector<int> indeg(numCourses, 0), order;
        for (auto& p : prerequisites) { adj[p[1]].push_back(p[0]); indeg[p[0]]++; }
        queue<int> q;
        for (int i = 0; i < numCourses; i++) if (indeg[i] == 0) q.push(i);
        while (!q.empty()) {
            int u = q.front(); q.pop();
            order.push_back(u);
            for (int v : adj[u]) if (--indeg[v] == 0) q.push(v);
        }
        if ((int)order.size() != numCourses) return {};
        return order;
    }
};

function findOrder(numCourses, prerequisites) {
  const adj = Array.from({ length: numCourses }, () => []), indeg = new Array(numCourses).fill(0);
  for (const [a, b] of prerequisites) { adj[b].push(a); indeg[a]++; }
  const order = [];
  for (let i = 0; i < numCourses; i++) if (indeg[i] === 0) order.push(i);
  for (let h = 0; h < order.length; h++)
    for (const v of adj[order[h]]) if (--indeg[v] === 0) order.push(v);
  return order.length === numCourses ? order : [];
}