Given a list of strings, group anagrams together. An anagram is a word formed by rearranging the letters of another word.

Your method should:

  • Group words that are anagrams of each other (case-insensitive comparison)
  • Return a List of List of Strings where each inner list contains anagram groups
  • Preserve the original case of the input words in the output
  • Return an empty list for empty input
  • Return null for null input

The order of groups in the output does not matter, and the order of words within each group does not matter.

Examples

Example 1: Basic Anagram Grouping

List<String> words = new List<String>{'eat', 'tea', 'tan', 'ate', 'nat', 'bat'};
List<List<String>> result = groupAnagrams(words);
// Returns: [['eat', 'tea', 'ate'], ['tan', 'nat'], ['bat']]

Example 2: Single Empty String

List<String> words = new List<String>{''};
List<List<String>> result = groupAnagrams(words);
// Returns: [['']]

Example 3: Empty List

List<String> words = new List<String>();
List<List<String>> result = groupAnagrams(words);
// Returns: []

Example 4: Null Input

List<String> words = null;
List<List<String>> result = groupAnagrams(words);
// Returns: null

Example 5: Case Preservation

List<String> words = new List<String>{'Eat', 'Tea', 'EAT'};
List<List<String>> result = groupAnagrams(words);
// Returns: [['Eat', 'Tea', 'EAT']]
// All are anagrams (case-insensitive), original case preserved
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