Common Elements in Two Lists | Dataford Interview Questions

GoDaddy may compare domain-name lists from different sources and needs the domain names that appear in both lists. Given two lists of strings, return the unique common elements, preserving the order in which they first appear in the first list.

Formal Specification

Implement common_elements(first, second).

first: a list of strings.
second: a list of strings.
Return a list containing each string that appears in both inputs exactly once.
The output order must match the first occurrence order in first.
Treat strings as case-sensitive, so "Example.com" and "example.com" are different values.

Constraints

0 <= len(first), len(second) <= 10^5
Each element is a non-empty string
Each string has length at most 253
The output contains unique values in first-list order
String comparison is case-sensitive

Function Signature

def common_elements(first, second):

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