diff --git a/.github/check_pr.py b/.github/check_pr.py
new file mode 100755
index 0000000..52546e0
--- /dev/null
+++ b/.github/check_pr.py
@@ -0,0 +1,204 @@
+#!/usr/bin/env python3
+"""PR gate: repo structure + the rules in CONTRIBUTING.md. Run from the repo root."""
+
+import argparse
+import ast
+import json
+import re
+import subprocess
+import sys
+from pathlib import Path
+
+# Every entry allowed at the repo root. Adding one is a structural change.
+TOP_LEVEL = {
+    ".github", ".gitignore", "CONTRIBUTING.md", "LICENSE", "README.md",
+    "requirements.txt", "data", "Decorators & Namespaces", "docs",
+    "notebooks", "resources", "scripts", "tests",
+}
+# Which extension belongs in which directory, and how deep a new file may sit.
+DIR_RULES = {"scripts": ".py", "notebooks": ".ipynb", "docs": ".md", "tests": ".py"}
+FREEFORM = {"data", "resources", "Decorators & Namespaces", ".github"}
+
+IMPORT_ALIASES = {
+    "cv2": "opencv-python", "PIL": "pillow", "sklearn": "scikit-learn",
+    "bs4": "beautifulsoup4", "yaml": "pyyaml", "dotenv": "python-dotenv",
+}
+LOCAL_PATH = re.compile(r"/Users/|/home/[a-z]|[A-Z]:\\Users")
+DESTRUCTIVE = re.compile(r"shutil\.(rmtree|move)|os\.(remove|unlink|rmdir|rename)|\.unlink\(|\.replace\(")
+
+errors, warnings = [], []
+
+
+def fail(path, msg):
+    errors.append(f"{path}: {msg}")
+
+
+def warn(path, msg):
+    warnings.append(f"{path}: {msg}")
+
+
+def changed_files(base):
+    out = subprocess.run(
+        ["git", "diff", "--name-status", "--find-renames", f"{base}...HEAD"],
+        capture_output=True, text=True, check=True,
+    ).stdout
+    changes = []
+    for line in out.splitlines():
+        parts = line.split("\t")
+        changes.append((parts[0][0], parts[1:]))
+    return changes
+
+
+def requirement_names():
+    names = set()
+    for line in Path("requirements.txt").read_text().splitlines():
+        line = line.split("#")[0].strip()
+        if line:
+            names.add(re.split(r"[<>=!\[;\s]", line)[0].lower().replace("_", "-"))
+    return names
+
+
+def imported_roots(tree):
+    roots = set()
+    for node in ast.walk(tree):
+        if isinstance(node, ast.Import):
+            roots.update(a.name.split(".")[0] for a in node.names)
+        elif isinstance(node, ast.ImportFrom) and not node.level and node.module:
+            roots.add(node.module.split(".")[0])
+    return roots
+
+
+def check_structure(changes):
+    for status, paths in changes:
+        target = paths[-1]
+        top = target.split("/")[0]
+        if status in "AR" and top not in TOP_LEVEL:
+            fail(target, f"new top-level entry '{top}'. Put it in one of: {', '.join(sorted(DIR_RULES))}")
+            continue
+        if status == "R":
+            fail(target, f"renamed/moved from '{paths[0]}'. Open an issue first — moves break links in the docs and notebooks")
+        if status == "D":
+            fail(target, "deleted. Deletions need a maintainer's sign-off; open an issue instead")
+        if status == "A" and top in DIR_RULES:
+            bits = target.split("/")
+            if len(bits) != 2:
+                fail(target, f"nested under {top}/. One flat file per contribution, no subdirectories")
+            elif not target.endswith(DIR_RULES[top]):
+                fail(target, f"{top}/ holds only {DIR_RULES[top]} files")
+
+
+def test_sources(_cache={}):
+    # Why: a script counts as checked if a test file names it
+    if not _cache:
+        files = list(Path("tests").glob("test_*.py")) + list(Path("scripts").glob("test_*.py"))
+        _cache["text"] = "\n".join(p.read_text(errors="replace") for p in files)
+    return _cache["text"]
+
+
+def check_script(path, reqs, repo_modules):
+    source = Path(path).read_text(encoding="utf-8", errors="replace")
+    try:
+        tree = ast.parse(source, filename=path)
+    except SyntaxError as exc:
+        fail(path, f"does not parse: line {exc.lineno}: {exc.msg}")
+        return
+
+    for root in sorted(imported_roots(tree)):
+        if root in sys.stdlib_module_names or root in repo_modules:
+            continue
+        pkg = IMPORT_ALIASES.get(root, root).lower().replace("_", "-")
+        if pkg not in reqs:
+            fail(path, f"imports third-party '{root}' — add '{pkg}' to requirements.txt and say why in a comment")
+
+    if DESTRUCTIVE.search(source) and "--dry-run" not in source:
+        fail(path, "moves or deletes files but has no --dry-run flag")
+
+    has_logic = any(isinstance(n, (ast.For, ast.While, ast.FunctionDef)) for n in ast.walk(tree))
+    stem = Path(path).stem
+    tested = Path(f"tests/test_{stem}.py").exists() or stem in test_sources()
+    if has_logic and "assert" not in source and not tested:
+        warn(path, "real logic but no check — add a few asserts under `if __name__ == \"__main__\":`")
+
+
+def check_notebook(path):
+    try:
+        nb = json.loads(Path(path).read_text(encoding="utf-8"))
+    except json.JSONDecodeError as exc:
+        fail(path, f"is not valid notebook JSON: {exc}")
+        return
+    size = 0
+    for cell in nb.get("cells", []):
+        blob = json.dumps(cell.get("outputs", []))
+        size += len(blob)
+        if LOCAL_PATH.search(blob):
+            fail(path, "output contains a local filesystem path — clear the outputs before committing")
+            return
+    if size > 1_000_000:
+        warn(path, f"{size // 1000} KB of saved output — clear the outputs to keep clones small")
+
+
+def main():
+    ap = argparse.ArgumentParser(description=__doc__)
+    ap.add_argument("--base", default="origin/main", help="branch or ref to diff against")
+    ap.add_argument("--selftest", action="store_true")
+    args = ap.parse_args()
+
+    if args.selftest:
+        selftest()
+        return 0
+
+    if not Path("CONTRIBUTING.md").exists():
+        print("Run this from the repo root.", file=sys.stderr)
+        return 2
+
+    try:
+        changes = changed_files(args.base)
+    except subprocess.CalledProcessError:
+        print(f"Cannot diff against '{args.base}'. Fetch it first: git fetch origin main", file=sys.stderr)
+        return 2
+
+    if not changes:
+        print(f"No changes against {args.base}.")
+        return 0
+
+    check_structure(changes)
+    reqs = requirement_names()
+    # Why: sibling scripts and the scripts/ and tests/ packages are local, not PyPI
+    repo_modules = {p.stem for p in Path("scripts").glob("*.py")} | set(DIR_RULES)
+    for status, paths in changes:
+        target = paths[-1]
+        if status == "D" or not Path(target).exists():
+            continue
+        if target.endswith(".py"):
+            check_script(target, reqs, repo_modules)
+        elif target.endswith(".ipynb"):
+            check_notebook(target)
+
+    for line in warnings:
+        print(f"warning: {line}")
+    for line in errors:
+        print(f"error:   {line}")
+    print(f"\n{len(changes)} changed file(s), {len(errors)} error(s), {len(warnings)} warning(s).")
+    if errors:
+        print("See CONTRIBUTING.md for the rule behind each error.")
+    return 1 if errors else 0
+
+
+def selftest():
+    tree = ast.parse("import os, numpy as np\nfrom pathlib import Path\nfrom . import x")
+    assert imported_roots(tree) == {"os", "numpy", "pathlib"}
+    assert re.split(r"[<>=!\[;\s]", "pdf2docx>=0.5")[0] == "pdf2docx"
+    assert LOCAL_PATH.search("/Users/someone/data.csv")
+    assert not LOCAL_PATH.search("data/sample.txt")
+    assert DESTRUCTIVE.search("shutil.rmtree(p)") and not DESTRUCTIVE.search("os.listdir(p)")
+    errors.clear()
+    check_structure([("A", ["tools/x.py"]), ("A", ["scripts/deep/x.py"]), ("A", ["scripts/a.txt"]), ("D", ["README.md"])])
+    assert len(errors) == 4, errors
+    errors.clear()
+    check_structure([("A", ["scripts/ok.py"]), ("M", ["README.md"])])
+    assert not errors, errors
+    print("selftest ok")
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/.github/workflows/pr-check.yml b/.github/workflows/pr-check.yml
new file mode 100644
index 0000000..f6c6119
--- /dev/null
+++ b/.github/workflows/pr-check.yml
@@ -0,0 +1,26 @@
+name: PR check
+
+on:
+  pull_request:
+    branches: [main]
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Structure and CONTRIBUTING rules
+        run: python .github/check_pr.py --base origin/${{ github.base_ref }}
+
+      - name: Every script compiles
+        run: python -m compileall -q scripts
+
+      - name: Tests
+        run: python -m unittest discover tests -v
diff --git a/requirements.txt b/requirements.txt
index ec909cf..fc9e5ff 100644
Binary files a/requirements.txt and b/requirements.txt differ
diff --git a/tests/test_algorithms.py b/tests/test_algorithms.py
new file mode 100644
index 0000000..fe17f94
--- /dev/null
+++ b/tests/test_algorithms.py
@@ -0,0 +1,101 @@
+"""Tests for the search and sort scripts.
+
+Run from the repo root:
+    python -m unittest discover tests
+"""
+
+import contextlib
+import importlib.util
+import io
+import unittest
+from pathlib import Path
+
+SCRIPTS = Path(__file__).resolve().parent.parent / "scripts"
+
+
+def load(filename):
+    # Why: names like "01.TwoSum.py" can't be imported with `from scripts.x import y`
+    spec = importlib.util.spec_from_file_location(filename, SCRIPTS / filename)
+    module = importlib.util.module_from_spec(spec)
+    with contextlib.redirect_stdout(io.StringIO()):  # Why: some scripts print at import
+        spec.loader.exec_module(module)
+    return module
+
+
+class TestBinarySearch(unittest.TestCase):
+    def setUp(self):
+        self.search = load("BinarySearch.py").binarySearch
+
+    def find(self, array, x):
+        return self.search(array, x, 0, len(array) - 1)
+
+    def test_finds_element(self):
+        array = [3, 4, 5, 6, 7, 8, 9]
+        for i, value in enumerate(array):
+            self.assertEqual(self.find(array, value), i)
+
+    def test_missing_element(self):
+        self.assertEqual(self.find([3, 4, 5], 99), -1)
+
+    def test_empty_array(self):
+        self.assertEqual(self.find([], 1), -1)
+
+
+class TestLinearSearch(unittest.TestCase):
+    def setUp(self):
+        self.search = load("LinearSearch.py").linearSearch
+
+    def test_finds_element(self):
+        array = [2, 4, 0, 1, 9]
+        self.assertEqual(self.search(array, len(array), 1), 3)
+
+    def test_returns_first_match(self):
+        array = [7, 7, 7]
+        self.assertEqual(self.search(array, len(array), 7), 0)
+
+    def test_missing_element(self):
+        self.assertEqual(self.search([2, 4], 2, 5), -1)
+
+
+class TestMergeSort(unittest.TestCase):
+    def setUp(self):
+        self.sort = load("MergeSort.py").mergeSort
+
+    def sorted_copy(self, array):
+        array = list(array)
+        self.sort(array)  # Why: sorts in place, returns None
+        return array
+
+    def test_sorts_in_place(self):
+        self.assertEqual(self.sorted_copy([6, 5, 12, 10, 9, 1]), [1, 5, 6, 9, 10, 12])
+
+    def test_duplicates_and_negatives(self):
+        self.assertEqual(self.sorted_copy([3, -1, 3, 0, -1]), [-1, -1, 0, 3, 3])
+
+    def test_short_inputs(self):
+        self.assertEqual(self.sorted_copy([]), [])
+        self.assertEqual(self.sorted_copy([42]), [42])
+        self.assertEqual(self.sorted_copy([2, 1]), [1, 2])
+
+
+class TestTwoSum(unittest.TestCase):
+    def setUp(self):
+        self.solve = load("01.TwoSum.py").Solution().twoSum
+
+    def test_returns_indices_of_pair(self):
+        nums, target = [2, 7, 11, 15], 9
+        i, j = self.solve(nums, target)
+        self.assertNotEqual(i, j)
+        self.assertEqual(nums[i] + nums[j], target)
+
+    def test_unsorted_input(self):
+        nums, target = [3, 2, 4], 6
+        i, j = self.solve(nums, target)
+        self.assertEqual(sorted([nums[i], nums[j]]), [2, 4])
+
+    def test_no_pair_returns_empty_list(self):
+        self.assertEqual(self.solve([1, 2, 3], 100), [])
+
+
+if __name__ == "__main__":
+    unittest.main()