diff --git a/03/README.md b/03/README.md
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+### Что здесь лежит?
+- в директории `merge` -- типизированный код с реализацией
+сортировки слиянием;
+- в директории `tests` -- соответственно, тесты;
+
+### Как запускать?
+
+Желательно установить [mypy](https://github.com/python/mypy) или даже
+[pyright](https://github.com/microsoft/pyright) и проверять типизацию
+ими перед запусками:
+```sh
+$ mypy --install-types --show-error-codes --pretty .
+```
+
+_*Выясните самостоятельно:*_
+что делают флаги `--install-types`, `--show-error-codes`, `--pretty`?
+
+Для запуска тестов:
+```sh
+$ python -m unittest discover -v tests/
+```
diff --git a/03/merge/__init__.py b/03/merge/__init__.py
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diff --git a/03/merge/imlementation.py b/03/merge/imlementation.py
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+++ b/03/merge/imlementation.py
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+from collections import deque
+from typing import List, Deque
+
+
+def merge(
+    els1: List[int],
+    els2: List[int],
+) -> List[int]:
+    """
+    Merges two sorted lists into one, keeps order.
+    >>> merge([1, 2, 3], [3, 4, 5])
+    [1, 2, 3, 3, 4, 5]
+    """
+    result = []
+    i, j = 0, 0
+    while i < len(els1) and j < len(els2):
+        if els1[i] < els2[j]:
+            result.append(els1[i])
+            i += 1
+        else:
+            result.append(els2[j])
+            j += 1
+
+    result.extend(els1[i:])
+    result.extend(els2[j:])
+    return result
+
+
+def mergesort(elements: List[int]) -> List[int]:
+    """
+    Sorts a given list.
+    """
+    # -> if len(elements) == 0:
+    # if not elements:
+    if len(elements) <= 1:
+        return elements
+
+    buffer: Deque[List[int]] = deque()
+    for el in elements:
+        buffer.append([el])
+
+    while len(buffer) > 1:
+        h1: List[int] = buffer.popleft()
+        h2: List[int] = buffer.popleft()
+        buffer.append(merge(h1, h2))
+
+    return buffer.pop()
diff --git a/03/tests/__init__.py b/03/tests/__init__.py
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diff --git a/03/tests/test_merge.py b/03/tests/test_merge.py
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+++ b/03/tests/test_merge.py
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+import unittest
+
+from merge.imlementation import merge, mergesort
+
+from typing import List
+
+
+class MergeTestCase(unittest.TestCase):
+    def test_merge_minimal(self):
+        self.assertEqual(
+            merge([], []),
+            [],
+        )
+
+    def test_merge_one(self):
+        self.assertEqual(merge([301], []), [301])
+        self.assertEqual(merge([-301], []), [-301])
+        self.assertEqual(merge([], [-31]), [-31])
+        self.assertEqual(merge([], [31]), [31])
+
+    def test_merge_two(self):
+        self.assertEqual(merge([301], [1]), [1, 301])
+        self.assertEqual(merge([1], [301]), [1, 301])
+        self.assertEqual(merge([-301, 1], []), [-301, 1])
+        self.assertEqual(merge([], [-301, 1]), [-301, 1])
+
+    def test_mergesort_minimal(self):
+        self.assertEqual(
+            mergesort([]),
+            [],
+        )
+
+    def test_mergesort_one(self):
+        self.assertEqual(mergesort([3]), [3])
+        self.assertEqual(mergesort([-3]), [-3])
+
+    def test_mergesort_two(self):
+        self.assertEqual(mergesort([1, 3]), [1, 3])
+        self.assertEqual(mergesort([3, 1]), [1, 3])
+        self.assertEqual(
+            mergesort([3, 3]),
+            [3, 3],
+        )
+
+    def test_small(self):
+        self.assertEqual(
+            mergesort([3, 3, 1, 2, 4, 6, 2, 4]),
+            [1, 2, 2, 3, 3, 4, 4, 6],
+        )
+
+    def test_small2(self):
+        self.assertEqual(
+            mergesort([3, -3, 1, 2, 4, -6, 2, 4]),
+            [-6, -3, 1, 2, 2, 3, 4, 4],
+        )
+
+    # def test_forbidden(self):
+    #     self.assertEqual(
+    #         mergesort(["a", "b", 3]),
+    #         ["a", "b", 3],
+    #     )