Alien Dictionary

def alienOrder(words):
    adj = {c: set() for w in words for c in w}
    
    for i in range(len(words) - 1):
        w1, w2 = words[i], words[i + 1]
        minLen = min(len(w1), len(w2))
        if len(w1) > len(w2) and w1[:minLen] == w2[:minLen]:
            return ""
        for j in range(minLen):
            if w1[j] != w2[j]:
                adj[w1[j]].add(w2[j])
                break
                
    visit = {} # False=visited, True=current path
    res = []
    
    def dfs(c):
        if c in visit:
            return visit[c]
        visit[c] = True
        for nei in adj[c]:
            if dfs(nei): return True
        visit[c] = False
        res.append(c)
        
    for c in adj:
        if dfs(c): return ""
    res.reverse()
    return "".join(res)