The topological sorting algorithm (corrected book version)

 INPUT: a digraph G with n vertices
 OUTPUT: a topological ordering v_1,v_2,..,v_n of G or an indication that G has a cycle

 let S be an empty stack
 FOR each vertex u of G DO
   incounter(u) <- indeg(u)
   IF incounter(u) == 0 THEN S.push(u)
 i <- 1
 WHILE S is not empty DO
   u <- S.pop()
   number u as the i-th vertex v_i of G
   i <- i+1
   FOR each edge e in G.outIncidentEdges(u) DO
     w <- G.opposite(u,e)
     incounter(w) <- incounter(w) - 1
     IF incounter(w) == 0; THEN S.push(w)

 // this final bit differs from the wrong book version:

 IF i>n THEN RETURN order   v_1,v_2,..,v_n
 ELSE RETURN "G must have a cycle"