Added tree algorithm

[VarnitRaina]

Description
This pull request introduces a new algorithm to generate all unique permutations of a given string, even when the string contains duplicate characters. The algorithm is implemented in the UniquePermutations class under the package com.thealgorithms.Recursion.

Key Features:
Algorithm Implementation:

The method getUniquePermutations(String str) generates all unique permutations of the input string.
The algorithm sorts the characters of the string to handle duplicate characters effectively.
It employs a backtracking approach to explore all possible arrangements of the characters.
Edge Case Handling:

The algorithm accounts for various edge cases such as:
Null Input: Returns an empty list.
Empty String: Returns a list containing an empty string as the only permutation.
Single Character: Returns a list containing the single character as the only permutation.
Identical Characters: Returns a list with one entry for strings composed of identical characters.
Test Cases:
Comprehensive test cases have been added to ensure the functionality and correctness of the algorithm. These test cases are located in the UniquePermutationsTest class and include:

Basic Tests: Validate the algorithm with simple strings like "abc".
Duplicate Characters: Check handling of duplicates with strings like "aab".
Edge Cases: Confirm behavior for empty strings, single characters, and strings with identical characters.
Mixed Case: Ensure the algorithm correctly handles mixed case strings like "aAb".
Null Input: Verify that null input results in an empty list.
Reference
For a simpler version of this algorithm, you can refer to this GeeksforGeeks article.

[siriak]

Please fix PR checks

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