Solution.py

class Solution:
    def alienOrder(self, words: List[str]) -> str:
        g = [[False] * 26 for _ in range(26)]
        s = [False] * 26
        cnt = 0
        n = len(words)
        for i in range(n - 1):
            for c in words[i]:
                if cnt == 26:
                    break
                o = ord(c) - ord('a')
                if not s[o]:
                    cnt += 1
                    s[o] = True
            m = len(words[i])
            for j in range(m):
                if j >= len(words[i + 1]):
                    return ''
                c1, c2 = words[i][j], words[i + 1][j]
                if c1 == c2:
                    continue
                o1, o2 = ord(c1) - ord('a'), ord(c2) - ord('a')
                if g[o2][o1]:
                    return ''
                g[o1][o2] = True
                break
        for c in words[n - 1]:
            if cnt == 26:
                break
            o = ord(c) - ord('a')
            if not s[o]:
                cnt += 1
                s[o] = True

        indegree = [0] * 26
        for i in range(26):
            for j in range(26):
                if i != j and s[i] and s[j] and g[i][j]:
                    indegree[j] += 1
        q = deque()
        ans = []
        for i in range(26):
            if s[i] and indegree[i] == 0:
                q.append(i)
        while q:
            t = q.popleft()
            ans.append(chr(t + ord('a')))
            for i in range(26):
                if s[i] and i != t and g[t][i]:
                    indegree[i] -= 1
                    if indegree[i] == 0:
                        q.append(i)
        return '' if len(ans) < cnt else ''.join(ans)