feat(algorithms, hash table): first unique character in string

DIRECTORY.md

@@ -166,6 +166,8 @@
     * Two City Scheduling
       * [Test Two City Scheduling](https://github.com/BrianLusina/PythonSnips/blob/master/algorithms/greedy/two_city_scheduling/test_two_city_scheduling.py)
   * Hash Table
+    * First Unique Character
+      * [Test First Unique Character](https://github.com/BrianLusina/PythonSnips/blob/master/algorithms/hash_table/first_unique_character/test_first_unique_character.py)
     * Ransom Note
       * [Test Ransom Note](https://github.com/BrianLusina/PythonSnips/blob/master/algorithms/hash_table/ransom_note/test_ransom_note.py)
   * Heap

algorithms/hash_table/first_unique_character/README.md

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+# First Unique Character
+
+Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1.
+
+## Examples
+
+Example 1:
+
+```text
+Input: s = "leetcode"
+Output: 0
+Explanation:
+The character 'l' at index 0 is the first character that does not occur at any other index.
+```
+
+Example 2:
+
+```text
+Input: s = "loveleetcode"
+Output: 2
+```
+
+Example 3:
+
+```text
+Input: s = "aabb"
+Output: -1
+```
+
+## Constraints
+
+- 1 <= `s.length` <= 10^5
+- `s` consists of lowercase English letters
+- There are no spaces in the string.
+

algorithms/hash_table/first_unique_character/__init__.py

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+from collections import Counter
+
+
+def first_unique_character(s: str) -> int:
+    frequency = Counter(s)
+
+    for idx, letter in enumerate(s):
+        if frequency[letter] == 1:
+            return idx
+    return -1

algorithms/hash_table/first_unique_character/test_first_unique_character.py

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+import unittest
+from parameterized import parameterized
+from algorithms.hash_table.first_unique_character import first_unique_character
+
+FIRST_UNIQUE_CHARACTER_TEST_CASES = [
+    ("leetcode", 0),
+    ("loveleetcode", 2),
+    ("aabb", -1),
+    ("baefeab", 3),
+    ("aabbcc", -1),
+    ("dajhfiuebdafsdhdgaj", 5),
+    ("xyurtwxwtryua", 12),
+    ("aeiouqwertyauieotweryqq", -1),
+    ("awsjuhfajwfnkag", 2),
+]
+
+
+class FirstUniqueCharacterTestCase(unittest.TestCase):
+    @parameterized.expand(FIRST_UNIQUE_CHARACTER_TEST_CASES)
+    def test_first_unique_character(self, s: str, expected: int):
+        actual = first_unique_character(s)
+        self.assertEqual(expected, actual)
+
+
+if __name__ == '__main__':
+    unittest.main()