How should I practice Infer a Lexicographically Smallest Alien Alphabet?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Microsoft phone screen and onsite interview problems in FastPrep.

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Infer a Lexicographically Smallest Alien Alphabet

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Problem statement

An alien language uses the lowercase English letters, but their order is unknown. You receive an array words sorted according to that language.

Infer a character order that is consistent with the dictionary. Include every distinct character that appears in words exactly once.

If several orders are valid, return the lexicographically smallest valid order.
If a longer word appears before its exact prefix, or the inferred precedence rules contain a cycle, return the empty string.

Function

alienOrder(words: String[]) → String

Examples

Example 1

words = ["wrt","wrf","er","ett","rftt"]return = "wertf"

The adjacent words imply w < e < r < t < f, so the order is unique.

Example 2

words = ["za","zb","ca","cb"]return = "abzc"

The rules require a < b and z < c; choosing the smallest available character at each step gives abzc.

Example 3

words = ["abc","ab"]return = ""

A longer word cannot precede its exact prefix in a valid sorted dictionary.

Constraints

1 <= words.length <= 100
1 <= words[i].length <= 100
Every word contains only lowercase English letters.
The total number of characters is at most 10^4.