diff --git a/requirements.txt b/requirements.txt
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+++ b/requirements.txt
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+pytest
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diff --git a/src/__pycache__/string_utils.cpython-312.pyc b/src/__pycache__/string_utils.cpython-312.pyc
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diff --git a/src/array_utils.py b/src/array_utils.py
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+++ b/src/array_utils.py
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+from typing import List, Union, Any
+
+def flatten_array(arr: Union[List, Any]) -> List:
+    """
+    Recursively flatten a nested list into a single-level list.
+    
+    Args:
+        arr (Union[List, Any]): Input list or nested list to be flattened.
+    
+    Returns:
+        List: A flattened list containing all non-list elements.
+    
+    Examples:
+        >>> flatten_array([1, [2, 3], [4, [5, 6]]])
+        [1, 2, 3, 4, 5, 6]
+        >>> flatten_array([1, 2, 3])
+        [1, 2, 3]
+        >>> flatten_array([])
+        []
+    """
+    # Base case: if input is not a list, return it as a single-element list
+    if not isinstance(arr, list):
+        return [arr]
+    
+    # Recursive case: flatten each element and combine
+    flattened = []
+    for item in arr:
+        # Recursively flatten each item and extend the result
+        flattened.extend(flatten_array(item))
+    
+    return flattened
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diff --git a/src/string_utils.py b/src/string_utils.py
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+++ b/src/string_utils.py
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+def reverse_string(input_string):
+    """
+    Reverse the given string using a manual character-by-character reversal algorithm.
+
+    Args:
+        input_string (str): The string to be reversed.
+
+    Returns:
+        str: The reversed string.
+
+    Raises:
+        TypeError: If the input is not a string.
+    """
+    # Check if input is a string
+    if not isinstance(input_string, str):
+        raise TypeError("Input must be a string")
+    
+    # Convert string to list of characters for manual reversal
+    chars = list(input_string)
+    
+    # Manual reversal using two-pointer technique
+    left = 0
+    right = len(chars) - 1
+    
+    while left < right:
+        # Swap characters
+        chars[left], chars[right] = chars[right], chars[left]
+        
+        # Move pointers towards center
+        left += 1
+        right -= 1
+    
+    # Convert back to string and return
+    return ''.join(chars)
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diff --git a/tests/__pycache__/test_string_utils.cpython-312-pytest-8.3.5.pyc b/tests/__pycache__/test_string_utils.cpython-312-pytest-8.3.5.pyc
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diff --git a/tests/test_array_utils.py b/tests/test_array_utils.py
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+++ b/tests/test_array_utils.py
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+import pytest
+from src.array_utils import flatten_array
+
+def test_flatten_simple_list():
+    """Test flattening a simple list."""
+    assert flatten_array([1, 2, 3]) == [1, 2, 3]
+
+def test_flatten_nested_list():
+    """Test flattening a nested list."""
+    assert flatten_array([1, [2, 3], [4, [5, 6]]]) == [1, 2, 3, 4, 5, 6]
+
+def test_flatten_deeply_nested_list():
+    """Test flattening a deeply nested list."""
+    assert flatten_array([1, [2, [3, [4]]], 5]) == [1, 2, 3, 4, 5]
+
+def test_flatten_empty_list():
+    """Test flattening an empty list."""
+    assert flatten_array([]) == []
+
+def test_flatten_mixed_content():
+    """Test flattening a list with mixed content types."""
+    assert flatten_array([1, 'a', [2, ['b']], 3]) == [1, 'a', 2, 'b', 3]
+
+def test_flatten_single_item():
+    """Test flattening a single item."""
+    assert flatten_array(42) == [42]
+
+def test_flatten_nested_empty_lists():
+    """Test flattening list with nested empty lists."""
+    assert flatten_array([[], [1, []], 2]) == [1, 2]
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diff --git a/tests/test_string_utils.py b/tests/test_string_utils.py
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index 0000000..63ba350
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+++ b/tests/test_string_utils.py
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+import pytest
+from src.string_utils import reverse_string
+
+def test_reverse_standard_string():
+    """Test reversing a standard ASCII string."""
+    assert reverse_string("hello") == "olleh"
+    assert reverse_string("python") == "nohtyp"
+
+def test_reverse_empty_string():
+    """Test reversing an empty string."""
+    assert reverse_string("") == ""
+
+def test_reverse_unicode_string():
+    """Test reversing a string with unicode characters."""
+    assert reverse_string("こんにちは") == "はちにんこ"
+    assert reverse_string("Hello, 世界!") == "!界世 ,olleH"
+
+def test_reverse_palindrome():
+    """Test reversing a palindrome."""
+    assert reverse_string("racecar") == "racecar"
+    assert reverse_string("12321") == "12321"
+
+def test_reverse_with_spaces_and_punctuation():
+    """Test reversing strings with spaces and punctuation."""
+    assert reverse_string("hello world") == "dlrow olleh"
+    assert reverse_string("a,b,c") == "c,b,a"
+
+def test_invalid_input_type():
+    """Test that non-string inputs raise a TypeError."""
+    with pytest.raises(TypeError):
+        reverse_string(12345)
+    
+    with pytest.raises(TypeError):
+        reverse_string(None)
+    
+    with pytest.raises(TypeError):
+        reverse_string(["hello"])
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