Problem Description

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.

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Examples

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"

解析

題目給定一個 字串陣列 words,

其中假定這些 words 中的字串是造著某一種字典序來判斷

也就是對兩個 word[i], word[j] 如果 i < j

必須是以下的可能

1. len(word[i]) < len(word[j])
2. minLen = min(len(word[i]), len(word[j])), 則在 index 介於 0 到 minLen 第一個 word[i][index] ≠ word[j][index] → word[i][index] < word[j][index]

要求要透過給定的 words 來找出這些字元的字典序排列

假設存在字典序的話，以最小字典序來回傳

然後把這些字元順序組成一個字串回傳

假設沒有則回傳 “”