Explain Valid Anagram and Group Anagrams.

Short answer: Valid Anagram: count character frequencies (or sort both strings). Group Anagrams: use a frequency signature or sorted string as dictionary key and bucket words together.

Complexity

Valid: O(n). Group: O(n * k log k) with sort key, or O(n * k) with count key (k = word length).

Common follow-ups

• Anagrams with Unicode
• Find if any two are anagrams in a stream

Prefer count[26] for lowercase English — faster and shows constraint awareness.