“对于程序员而言，刷了Leetcode不一定能拿offer，但是不刷肯定拿不到offer。”

Lusion鲁迅

DFS-Review

$$
T(n)=\Theta(V+E)
$$

DFS(G)
  for each vertex u in G.V
    u.color = WHITE
    u.p = NIL
  time = 0
  for each vertex u in G.V
  	if u.color == WHITHE
  		DFS-VISIT(G,u)
  		
DFS-VISIT(G,u)
	time = time + 1
	u.d = time
	u.color = GRAY
	for vertex v in G:Adj[u]
		if v.color == WHITE
			v.p = u
			DFS-VISIT(G,v)
	u.color = BLACK
	time = time + 1
	u.f = time

TOPOLOGICAL-SORT(G)
	DFS(G)
	list add each vertex v which is in G.V sorted by desc of v.f
	return list