Longest Flight Route - Implementation

Here's the solution:

function longestRoute(n, edges):
    adj := array of lists, size n + 1
    indegree := array of size n + 1, initialized to 0
    dist := array of size n + 1, initialized to -infinity
    parent := array of size n + 1, initialized to -1
    queue := empty queue
    topoOrder := empty list

    for each edge (a, b) in edges:
        add b to adj[a]
        indegree[b] := indegree[b] + 1

    for i from 1 to n:
        if indegree[i] = 0 then
            push i to queue

    while queue is not empty:
        u := pop from queue
        add u to topoOrder
        for each v in adj[u]:
            indegree[v] := indegree[v] - 1
            if indegree[v] = 0 then
                push v to queue

    dist[1] := 0

    for each u in topoOrder:
        if dist[u] ≠ -infinity then
            for each v in adj[u]:
                if dist[u] + 1 > dist[v] then
                    dist[v] := dist[u] + 1
                    parent[v] := u

    if dist[n] = -infinity then
        print "IMPOSSIBLE"
    else
        path := reconstruct path from 1 to n using parent array
        print length of path
        print path

Time: $O(V + E)$. Space: $O(V + E)$.