Topological Sort

public List<Integer> topoSort(int numNodes, int[][] edges) {
    int[] inDegree = new int[numNodes];
    Map<Integer, List<Integer>> adj = new HashMap<>();
    
    // Build Graph and calculate In-Degrees (how many prerequisites a node has)
    for (int[] edge : edges) {
        adj.computeIfAbsent(edge[0], k -> new ArrayList<>()).add(edge[1]);
        inDegree[edge[1]]++;
    }
    
    // Queue holds nodes with NO prerequisites
    Queue<Integer> q = new LinkedList<>();
    for (int i = 0; i < numNodes; i++) {
        if (inDegree[i] == 0) q.offer(i);
    }
    
    List<Integer> order = new ArrayList<>();
    while (!q.isEmpty()) {
        int curr = q.poll();
        order.add(curr);
        
        // Fulfill prerequisite for neighbors
        for (int neighbor : adj.getOrDefault(curr, new ArrayList<>())) {
            inDegree[neighbor]--;
            if (inDegree[neighbor] == 0) q.offer(neighbor); // Ready to process
        }
    }
    
    // If order size != numNodes, the graph has a cycle (impossible to complete)
    return order.size() == numNodes ? order : new ArrayList<>();
}