diff --git a/.gitignore b/.gitignore
index 63040b6..3b44555 100644
--- a/.gitignore
+++ b/.gitignore
@@ -20,7 +20,6 @@ uv.lock
 *.swo
 
 # Project specific
-problems/
 .solved.json
 
 # OS
diff --git a/problems/easy/palindrome-checker/solution.py b/problems/easy/palindrome-checker/solution.py
new file mode 100644
index 0000000..f6ef597
--- /dev/null
+++ b/problems/easy/palindrome-checker/solution.py
@@ -0,0 +1,33 @@
+"""
+Palindrome Checker
+
+You're building a word game that awards bonus points for palindromes.
+Given a string, determine if it reads the same forwards and backwards,
+ignoring case and non-alphanumeric characters.
+
+Example 1:
+    Input: text = "A man, a plan, a canal: Panama"
+    Output: True
+    Explanation: "amanaplanacanalpanama" is a palindrome
+
+Example 2:
+    Input: text = "race a car"
+    Output: False
+    Explanation: "raceacar" is not a palindrome
+
+Example 3:
+    Input: text = " "
+    Output: True
+    Explanation: Empty after removing non-alphanumeric
+
+Constraints:
+    - Input is always a string
+    - Ignore spaces, punctuation, and case
+    - Empty string is considered a palindrome
+"""
+
+
+def is_palindrome(text: str) -> bool:
+    """Return True if text is a palindrome, False otherwise."""
+    pass  # Your implementation here
+
diff --git a/problems/easy/palindrome-checker/tests.py b/problems/easy/palindrome-checker/tests.py
new file mode 100644
index 0000000..63bda68
--- /dev/null
+++ b/problems/easy/palindrome-checker/tests.py
@@ -0,0 +1,44 @@
+"""Tests for palindrome-checker."""
+import pytest
+from solution import is_palindrome
+
+
+class TestBasicCases:
+    """Test basic functionality with typical inputs."""
+
+    def test_simple_palindrome(self):
+        """Test basic palindrome word."""
+        assert is_palindrome("racecar") == True
+
+    def test_sentence_palindrome(self):
+        """Test palindrome with spaces and punctuation."""
+        assert is_palindrome("A man, a plan, a canal: Panama") == True
+
+    def test_not_palindrome(self):
+        """Test non-palindrome string."""
+        assert is_palindrome("hello") == False
+
+
+class TestEdgeCases:
+    """Test edge cases and boundary conditions."""
+
+    def test_empty_string(self):
+        """Test with empty input."""
+        assert is_palindrome("") == True
+
+    def test_single_character(self):
+        """Test with single character."""
+        assert is_palindrome("a") == True
+
+    def test_only_spaces(self):
+        """Test with only whitespace."""
+        assert is_palindrome("   ") == True
+
+    def test_mixed_case(self):
+        """Test case insensitivity."""
+        assert is_palindrome("RaceCar") == True
+
+    def test_numbers_in_string(self):
+        """Test with numbers."""
+        assert is_palindrome("12321") == True
+
diff --git a/problems/easy/two-sum/solution.py b/problems/easy/two-sum/solution.py
new file mode 100644
index 0000000..2cb606b
--- /dev/null
+++ b/problems/easy/two-sum/solution.py
@@ -0,0 +1,31 @@
+"""
+Two Sum
+
+You are a cashier at a busy store. A customer wants to pay exactly
+a target amount using exactly two items from their basket.
+
+Given a list of item prices and a target total, find the indices
+of the two items that add up to the target.
+
+Example 1:
+    Input: prices = [2, 7, 11, 15], target = 9
+    Output: [0, 1]
+    Explanation: prices[0] + prices[1] = 2 + 7 = 9
+
+Example 2:
+    Input: prices = [3, 2, 4], target = 6
+    Output: [1, 2]
+
+Constraints:
+    - 2 <= len(prices) <= 10^4
+    - Each price is a positive integer
+    - Exactly one solution exists
+"""
+
+
+def two_sum(prices: list[int], target: int) -> list[int]:
+    """Return indices of two prices that add up to target."""
+    for i in range(len(prices)):
+        for j in range(i+1, len(prices)):
+            if prices[i] + prices[j] == target:
+                return [i,j]
diff --git a/problems/easy/two-sum/tests.py b/problems/easy/two-sum/tests.py
new file mode 100644
index 0000000..83a5d46
--- /dev/null
+++ b/problems/easy/two-sum/tests.py
@@ -0,0 +1,35 @@
+"""Tests for two-sum."""
+import pytest
+from solution import two_sum
+
+
+class TestBasicCases:
+    """Test basic functionality with typical inputs."""
+
+    def test_example_one(self):
+        """Test first example from problem description."""
+        assert sorted(two_sum([2, 7, 11, 15], 9)) == [0, 1]
+
+    def test_example_two(self):
+        """Test second example from problem description."""
+        assert sorted(two_sum([3, 2, 4], 6)) == [1, 2]
+
+    def test_adjacent_elements(self):
+        """Test when answer elements are adjacent."""
+        assert sorted(two_sum([1, 2, 3, 4], 7)) == [2, 3]
+
+
+class TestEdgeCases:
+    """Test edge cases and boundary conditions."""
+
+    def test_same_value_twice(self):
+        """Test with duplicate values that sum to target."""
+        assert sorted(two_sum([3, 3], 6)) == [0, 1]
+
+    def test_first_and_last(self):
+        """Test when answer is first and last elements."""
+        assert sorted(two_sum([2, 4, 6, 8, 10, 7], 9)) == [0, 5]
+
+    def test_negative_numbers(self):
+        """Test with negative numbers in the list."""
+        assert sorted(two_sum([-1, -2, -3, -4, -5], -8)) == [2, 4]
diff --git a/problems/hard/lru-cache/solution.py b/problems/hard/lru-cache/solution.py
new file mode 100644
index 0000000..4c3c20f
--- /dev/null
+++ b/problems/hard/lru-cache/solution.py
@@ -0,0 +1,47 @@
+"""
+LRU Cache
+
+You're building a caching layer for a database-heavy web application.
+To reduce load, implement a Least Recently Used (LRU) cache that
+automatically evicts the oldest unused entries when capacity is reached.
+
+Example 1:
+    cache = LRUCache(capacity=2)
+    cache.put("a", 1)
+    cache.put("b", 2)
+    cache.get("a")       # Returns 1 (a is now most recently used)
+    cache.put("c", 3)    # Evicts "b" (least recently used)
+    cache.get("b")       # Returns -1 (not found)
+    cache.get("c")       # Returns 3
+
+Example 2:
+    cache = LRUCache(capacity=1)
+    cache.put("x", 10)
+    cache.put("y", 20)   # Evicts "x"
+    cache.get("x")       # Returns -1
+    cache.get("y")       # Returns 20
+
+Constraints:
+    - capacity >= 1
+    - Keys are strings, values are integers
+    - get() returns -1 if key not found
+    - put() updates value if key exists (and marks as recently used)
+    - Both get() and put() should be O(1) average time
+"""
+
+
+class LRUCache:
+    """Least Recently Used cache with O(1) operations."""
+
+    def __init__(self, capacity: int):
+        """Initialize cache with given capacity."""
+        pass  # Your implementation here
+
+    def get(self, key: str) -> int:
+        """Return value for key, or -1 if not found."""
+        pass  # Your implementation here
+
+    def put(self, key: str, value: int) -> None:
+        """Insert or update key-value pair, evicting LRU if needed."""
+        pass  # Your implementation here
+
diff --git a/problems/hard/lru-cache/tests.py b/problems/hard/lru-cache/tests.py
new file mode 100644
index 0000000..c3655a3
--- /dev/null
+++ b/problems/hard/lru-cache/tests.py
@@ -0,0 +1,86 @@
+"""Tests for lru-cache."""
+import pytest
+from solution import LRUCache
+
+
+class TestBasicCases:
+    """Test basic functionality with typical inputs."""
+
+    def test_basic_put_get(self):
+        """Test basic put and get operations."""
+        cache = LRUCache(2)
+        cache.put("a", 1)
+        cache.put("b", 2)
+        assert cache.get("a") == 1
+        assert cache.get("b") == 2
+
+    def test_eviction_lru(self):
+        """Test that least recently used is evicted."""
+        cache = LRUCache(2)
+        cache.put("a", 1)
+        cache.put("b", 2)
+        cache.get("a")        # a is now most recent
+        cache.put("c", 3)     # b should be evicted
+        assert cache.get("b") == -1
+        assert cache.get("a") == 1
+        assert cache.get("c") == 3
+
+    def test_update_existing_key(self):
+        """Test updating an existing key."""
+        cache = LRUCache(2)
+        cache.put("a", 1)
+        cache.put("a", 10)
+        assert cache.get("a") == 10
+
+
+class TestEdgeCases:
+    """Test edge cases and boundary conditions."""
+
+    def test_capacity_one(self):
+        """Test with capacity of 1."""
+        cache = LRUCache(1)
+        cache.put("a", 1)
+        cache.put("b", 2)
+        assert cache.get("a") == -1
+        assert cache.get("b") == 2
+
+    def test_get_nonexistent(self):
+        """Test getting a key that doesn't exist."""
+        cache = LRUCache(2)
+        assert cache.get("missing") == -1
+
+    def test_put_updates_recency(self):
+        """Test that put on existing key updates recency."""
+        cache = LRUCache(2)
+        cache.put("a", 1)
+        cache.put("b", 2)
+        cache.put("a", 10)    # a is now most recent
+        cache.put("c", 3)     # b should be evicted
+        assert cache.get("b") == -1
+        assert cache.get("a") == 10
+
+    def test_many_operations(self):
+        """Test a sequence of many operations."""
+        cache = LRUCache(3)
+        cache.put("a", 1)
+        cache.put("b", 2)
+        cache.put("c", 3)
+        cache.get("a")
+        cache.put("d", 4)     # evicts b
+        cache.put("e", 5)     # evicts c
+        assert cache.get("a") == 1
+        assert cache.get("b") == -1
+        assert cache.get("c") == -1
+        assert cache.get("d") == 4
+        assert cache.get("e") == 5
+
+    def test_get_updates_recency(self):
+        """Test that get updates the access order."""
+        cache = LRUCache(2)
+        cache.put("a", 1)
+        cache.put("b", 2)
+        cache.get("a")        # touch a
+        cache.put("c", 3)     # evicts b, not a
+        assert cache.get("a") == 1
+        assert cache.get("b") == -1
+
diff --git a/problems/hard/rate-limiter/solution.py b/problems/hard/rate-limiter/solution.py
new file mode 100644
index 0000000..b2e6b3f
--- /dev/null
+++ b/problems/hard/rate-limiter/solution.py
@@ -0,0 +1,49 @@
+"""
+Rate Limiter
+
+You're building an API gateway for a SaaS platform. To prevent abuse
+and ensure fair usage, you need to implement a rate limiter that tracks
+requests per user using a sliding window algorithm.
+
+The limiter should allow at most `max_requests` per user within any
+`window_seconds` time period.
+
+Example 1:
+    limiter = RateLimiter(max_requests=3, window_seconds=60)
+    limiter.allow_request("user1", timestamp=0)   # True (1st request)
+    limiter.allow_request("user1", timestamp=30)  # True (2nd request)
+    limiter.allow_request("user1", timestamp=45)  # True (3rd request)
+    limiter.allow_request("user1", timestamp=50)  # False (limit reached)
+    limiter.allow_request("user1", timestamp=61)  # True (1st expired)
+
+Example 2:
+    limiter = RateLimiter(max_requests=2, window_seconds=10)
+    limiter.allow_request("user1", timestamp=0)   # True
+    limiter.allow_request("user2", timestamp=0)   # True (different user)
+    limiter.allow_request("user1", timestamp=5)   # True
+    limiter.allow_request("user1", timestamp=8)   # False
+
+Constraints:
+    - max_requests >= 1
+    - window_seconds >= 1
+    - Timestamps are non-negative integers (seconds)
+    - Timestamps are non-decreasing per user
+    - user_id is a non-empty string
+"""
+
+
+class RateLimiter:
+    """Sliding window rate limiter for API request throttling."""
+
+    def __init__(self, max_requests: int, window_seconds: int):
+        """Initialize with request limit and time window."""
+        pass  # Your implementation here
+
+    def allow_request(self, user_id: str, timestamp: int) -> bool:
+        """Return True if request allowed, False if rate limited."""
+        pass  # Your implementation here
+
+    def get_remaining(self, user_id: str, timestamp: int) -> int:
+        """Return remaining requests allowed for user at timestamp."""
+        pass  # Your implementation here
+
diff --git a/problems/hard/rate-limiter/tests.py b/problems/hard/rate-limiter/tests.py
new file mode 100644
index 0000000..2331e9e
--- /dev/null
+++ b/problems/hard/rate-limiter/tests.py
@@ -0,0 +1,70 @@
+"""Tests for rate-limiter."""
+import pytest
+from solution import RateLimiter
+
+
+class TestBasicCases:
+    """Test basic functionality with typical inputs."""
+
+    def test_allow_within_limit(self):
+        """Test requests within the limit are allowed."""
+        limiter = RateLimiter(max_requests=3, window_seconds=60)
+        assert limiter.allow_request("user1", 0) == True
+        assert limiter.allow_request("user1", 30) == True
+        assert limiter.allow_request("user1", 45) == True
+
+    def test_block_over_limit(self):
+        """Test requests over limit are blocked."""
+        limiter = RateLimiter(max_requests=2, window_seconds=60)
+        assert limiter.allow_request("user1", 0) == True
+        assert limiter.allow_request("user1", 30) == True
+        assert limiter.allow_request("user1", 45) == False
+
+    def test_multiple_users_independent(self):
+        """Test each user has independent limits."""
+        limiter = RateLimiter(max_requests=1, window_seconds=60)
+        assert limiter.allow_request("user1", 0) == True
+        assert limiter.allow_request("user2", 0) == True
+        assert limiter.allow_request("user1", 30) == False
+        assert limiter.allow_request("user2", 30) == False
+
+    def test_get_remaining_basic(self):
+        """Test remaining count decreases with requests."""
+        limiter = RateLimiter(max_requests=3, window_seconds=60)
+        assert limiter.get_remaining("user1", 0) == 3
+        limiter.allow_request("user1", 0)
+        assert limiter.get_remaining("user1", 0) == 2
+
+
+class TestEdgeCases:
+    """Test edge cases and boundary conditions."""
+
+    def test_window_expiration(self):
+        """Test old requests expire from window."""
+        limiter = RateLimiter(max_requests=2, window_seconds=60)
+        assert limiter.allow_request("user1", 0) == True
+        assert limiter.allow_request("user1", 30) == True
+        assert limiter.allow_request("user1", 45) == False
+        assert limiter.allow_request("user1", 61) == True
+
+    def test_single_request_limit(self):
+        """Test with limit of 1 request."""
+        limiter = RateLimiter(max_requests=1, window_seconds=10)
+        assert limiter.allow_request("user1", 0) == True
+        assert limiter.allow_request("user1", 5) == False
+        assert limiter.allow_request("user1", 11) == True
+
+    def test_new_user_full_allowance(self):
+        """Test new users start with full allowance."""
+        limiter = RateLimiter(max_requests=5, window_seconds=60)
+        limiter.allow_request("user1", 0)
+        assert limiter.get_remaining("new_user", 20) == 5
+
+    def test_rapid_same_timestamp(self):
+        """Test multiple requests at same timestamp."""
+        limiter = RateLimiter(max_requests=3, window_seconds=1)
+        assert limiter.allow_request("user1", 0) == True
+        assert limiter.allow_request("user1", 0) == True
+        assert limiter.allow_request("user1", 0) == True
+        assert limiter.allow_request("user1", 0) == False
+
diff --git a/problems/medium/group-transactions/solution.py b/problems/medium/group-transactions/solution.py
new file mode 100644
index 0000000..7b4a1b3
--- /dev/null
+++ b/problems/medium/group-transactions/solution.py
@@ -0,0 +1,33 @@
+"""
+Transaction Grouper
+
+You're building a financial dashboard for a budgeting app. Users want
+to see their spending grouped by category with totals, so they can
+understand where their money is going each month.
+
+Example 1:
+    Input: transactions = [
+        {"amount": 50, "category": "food", "date": "2024-01-01"},
+        {"amount": 30, "category": "food", "date": "2024-01-02"},
+        {"amount": 100, "category": "transport", "date": "2024-01-01"}
+    ]
+    Output: {"food": 80, "transport": 100}
+    Explanation: food: 50+30=80, transport: 100
+
+Example 2:
+    Input: transactions = []
+    Output: {}
+    Explanation: No transactions means empty result
+
+Constraints:
+    - Each transaction has "amount" (positive int), "category" (str), "date" (str)
+    - Categories are case-sensitive
+    - Return categories in any order
+    - Amount is always positive
+"""
+
+
+def group_transactions(transactions: list[dict]) -> dict[str, int]:
+    """Return dictionary mapping each category to its total amount."""
+    pass  # Your implementation here
+
diff --git a/problems/medium/group-transactions/tests.py b/problems/medium/group-transactions/tests.py
new file mode 100644
index 0000000..32d8cbd
--- /dev/null
+++ b/problems/medium/group-transactions/tests.py
@@ -0,0 +1,58 @@
+"""Tests for group-transactions."""
+import pytest
+from solution import group_transactions
+
+
+class TestBasicCases:
+    """Test basic functionality with typical inputs."""
+
+    def test_multiple_categories(self):
+        """Test grouping across different categories."""
+        txns = [
+            {"amount": 50, "category": "food", "date": "2024-01-01"},
+            {"amount": 30, "category": "food", "date": "2024-01-02"},
+            {"amount": 100, "category": "transport", "date": "2024-01-01"}
+        ]
+        assert group_transactions(txns) == {"food": 80, "transport": 100}
+
+    def test_single_category(self):
+        """Test all transactions in one category."""
+        txns = [
+            {"amount": 10, "category": "food", "date": "2024-01-01"},
+            {"amount": 20, "category": "food", "date": "2024-01-02"},
+            {"amount": 30, "category": "food", "date": "2024-01-03"}
+        ]
+        assert group_transactions(txns) == {"food": 60}
+
+    def test_single_transaction(self):
+        """Test with just one transaction."""
+        txns = [{"amount": 25, "category": "entertainment", "date": "2024-01-01"}]
+        assert group_transactions(txns) == {"entertainment": 25}
+
+
+class TestEdgeCases:
+    """Test edge cases and boundary conditions."""
+
+    def test_empty_list(self):
+        """Test with no transactions."""
+        assert group_transactions([]) == {}
+
+    def test_case_sensitive_categories(self):
+        """Test that categories are case-sensitive."""
+        txns = [
+            {"amount": 10, "category": "Food", "date": "2024-01-01"},
+            {"amount": 20, "category": "food", "date": "2024-01-02"}
+        ]
+        result = group_transactions(txns)
+        assert result == {"Food": 10, "food": 20}
+
+    def test_many_categories(self):
+        """Test with many different categories."""
+        txns = [
+            {"amount": 1, "category": "a", "date": "2024-01-01"},
+            {"amount": 2, "category": "b", "date": "2024-01-01"},
+            {"amount": 3, "category": "c", "date": "2024-01-01"},
+            {"amount": 4, "category": "d", "date": "2024-01-01"}
+        ]
+        assert group_transactions(txns) == {"a": 1, "b": 2, "c": 3, "d": 4}
+
diff --git a/problems/medium/word-frequency/solution.py b/problems/medium/word-frequency/solution.py
new file mode 100644
index 0000000..6ae229e
--- /dev/null
+++ b/problems/medium/word-frequency/solution.py
@@ -0,0 +1,32 @@
+"""
+Word Frequency Counter
+
+You're building a text analytics tool for a content marketing team.
+Given a block of text, return the top N most frequently used words,
+sorted by frequency (highest first), then alphabetically for ties.
+
+Example 1:
+    Input: text = "the quick brown fox jumps over the lazy dog the fox"
+           n = 2
+    Output: [("the", 3), ("fox", 2)]
+    Explanation: "the" appears 3 times, "fox" appears 2 times
+
+Example 2:
+    Input: text = "hello world hello"
+           n = 5
+    Output: [("hello", 2), ("world", 1)]
+    Explanation: Only 2 unique words, return all of them
+
+Constraints:
+    - Words are separated by whitespace
+    - Case-insensitive (convert to lowercase)
+    - Ignore punctuation attached to words
+    - n >= 1
+    - If fewer than n unique words exist, return all of them
+"""
+
+
+def top_words(text: str, n: int) -> list[tuple[str, int]]:
+    """Return top n words by frequency as list of (word, count) tuples."""
+    pass  # Your implementation here
+
diff --git a/problems/medium/word-frequency/tests.py b/problems/medium/word-frequency/tests.py
new file mode 100644
index 0000000..eebf55c
--- /dev/null
+++ b/problems/medium/word-frequency/tests.py
@@ -0,0 +1,52 @@
+"""Tests for word-frequency."""
+import pytest
+from solution import top_words
+
+
+class TestBasicCases:
+    """Test basic functionality with typical inputs."""
+
+    def test_basic_frequency(self):
+        """Test basic word counting."""
+        result = top_words("the quick brown fox jumps over the lazy dog the fox", 2)
+        assert result == [("the", 3), ("fox", 2)]
+
+    def test_all_unique(self):
+        """Test when all words are unique."""
+        result = top_words("one two three", 2)
+        assert result == [("one", 1), ("three", 1)] or result == [("one", 1), ("two", 1)]
+
+    def test_single_word_repeated(self):
+        """Test with one word repeated."""
+        result = top_words("hello hello hello", 1)
+        assert result == [("hello", 3)]
+
+
+class TestEdgeCases:
+    """Test edge cases and boundary conditions."""
+
+    def test_empty_string(self):
+        """Test with empty input."""
+        result = top_words("", 5)
+        assert result == []
+
+    def test_n_greater_than_unique_words(self):
+        """Test when n exceeds unique word count."""
+        result = top_words("hello world", 10)
+        assert len(result) == 2
+
+    def test_case_insensitive(self):
+        """Test that counting is case-insensitive."""
+        result = top_words("Hello HELLO hello", 1)
+        assert result == [("hello", 3)]
+
+    def test_punctuation_ignored(self):
+        """Test that punctuation is stripped."""
+        result = top_words("hello, world! hello.", 1)
+        assert result == [("hello", 2)]
+
+    def test_alphabetical_tiebreaker(self):
+        """Test alphabetical ordering for same frequency."""
+        result = top_words("cat bat ant", 3)
+        assert result == [("ant", 1), ("bat", 1), ("cat", 1)]
+