Graph DFS

visited = set()
def dfs(u):
    if u == destination:
        return True
    
    if u in visited:
        return False
    
    visited.add(u)
    
    for v in adj[u]:
        if (dfs(v)):
            return True
    
    return False

return dfs(source)

stack = [source]
visited = set()
while stack:
    u = stack.pop()
    
    if u == destination:
        return True
    
    if u in visited:
        continue
    
    visited.add(u)
    
    for v in adj[u]:
        stack.append(v)

return False

adj = defaultdict(list)
for u, v in tickets:
    adj[u].append(v)

for u, nei in adj.items():
    nei.sort(reverse=True)

res = []
def dfs(u):
    nonlocal res
    while adj[u]:
        nxt = adj[u].pop()
        dfs(nxt)
    res.append(u)
dfs('JFK')
return res[::-1]

adj = defaultdict(list)
for u, v in edges:
    adj[u].append(v)

white, gray, black = 0, 1, 2
colors = [white] * n
def dfs(u):
    nonlocal white, gray, black

    if colors[u] == black:
        return True

    if colors[u] == gray:
        # loop
        return False

    if not adj[u]:
        # leaf
        return u == destination

    colors[u] = gray
    for v in adj[u]:
        ret = dfs(v)
        if not ret:
            return False
    colors[u] = black
    return True
    
return dfs(source)