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Leetcode 61. Alien Dictionary. Python

 

There is a new alien language which uses the latin alphabet. However, the order among letters are unknown to you. You receive a list of non-empty words from the dictionary, where words are sorted lexicographically by the rules of this new language. Derive the order of letters in this language.

Example

Example 1:

Input:["wrt","wrf","er","ett","rftt"]
Output:"wertf"
Explanation:
from "wrt"and"wrf" ,we can get 't'<'f'
from "wrt"and"er" ,we can get 'w'<'e'
from "er"and"ett" ,we can get 'r'<'t'
from "ett"and"rftt" ,we can get 'e'<'r'
So return "wertf"

Example 2:

Input:["z","x"]
Output:"zx"
Explanation:
from "z" and "x",we can get 'z' < 'x'
So return "zx"
Solution :
from typing import (
    List,
)

class Solution:
    def alien_order(self, words: List[str]) -> str:
        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]
            lmin = min(len(w1), len(w2))
            if len(w1) > len(w2) and w1[:lmin] == w2[:lmin] :
                return ""
            for j in range(lmin):
                if w1[j] != w2[j]:
                    adj[w1[j]].add(w2[j])
                    break
       
        ans = []
        vs = {}
       
        def dfs(c):
            if c in vs:
                return vs[c]

            vs[c] = True

            for n in adj[c]:
                if dfs(n):
                    return True

            vs[c] = False

            ans.append(c)
       
        for c in adj:
            if dfs(c) :
                return ""
               
        ans.reverse()

        return "".join(ans)


       

Explaination :







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