Find Recurring Names

Problem Overview

• Task: Given realNames and allNames, identify real users who registered multiple accounts using permutations of their names.
• Input/Output: Return distinct real names with duplicates in lexicographical order; if none, return "None".
• Constraints: n,m up to 1e5; names up to 10 chars; lowercase; realNames unique; no two are permutations; each allNames maps to a real name.
• Context: Detect coupon abuse via username permutations.
• Source: From Amazon interviews; a coding interview problem and common interview question.