diff --git a/counting-bits/Leo.py b/counting-bits/Leo.py
new file mode 100644
index 000000000..ec1142d5d
--- /dev/null
+++ b/counting-bits/Leo.py
@@ -0,0 +1,11 @@
+class Solution:
+    def countBits(self, n: int) -> List[int]:
+        counter = [0]
+
+        for i in range(1, n + 1):
+            counter.append(counter[i >> 1] + i % 2)
+
+        return counter
+
+        ## TC: O(n), SC: O(n)
+        ## this question should be under math section tbh
diff --git a/group-anagrams/Leo.py b/group-anagrams/Leo.py
new file mode 100644
index 000000000..3aa4f9822
--- /dev/null
+++ b/group-anagrams/Leo.py
@@ -0,0 +1,22 @@
+class Solution:
+    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
+        ans = collections.defaultdict(list)
+
+        for s in strs:
+            ans[str(sorted(s))].append(s)
+
+        return list(ans.values())
+
+        ## TC: O(n * klogk), SC: O(n * k), where n is len(strs) and k is len(longest_s)
+
+        # ans = {}
+
+        # for s in strs:
+        #     sorted_s = ''.join(sorted(s))
+
+        #     if sorted_s not in ans:
+        #         ans[sorted_s] = []
+
+        #     ans[sorted_s].append(s)
+
+        # return list(ans.values())
diff --git a/missing-number/Leo.py b/missing-number/Leo.py
new file mode 100644
index 000000000..1e6d30127
--- /dev/null
+++ b/missing-number/Leo.py
@@ -0,0 +1,8 @@
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        n = len(nums)
+
+        return int(n * (n + 1) / 2 - sum(nums))
+
+        # TC: O(n), SC: O(1)
+        # this only works for "only missing number", if there are multiple missing numbers, this won't work
diff --git a/number-of-1-bits/Leo.py b/number-of-1-bits/Leo.py
new file mode 100644
index 000000000..2675c6b02
--- /dev/null
+++ b/number-of-1-bits/Leo.py
@@ -0,0 +1,19 @@
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        counter = 0
+
+        while n:
+            counter += 1
+            n = n & (n - 1)
+
+        return counter
+
+        ## TC: O(k), SC: O(1), since given int(n) has constant length
+
+        # counter = 0
+
+        # for i in bin(n):
+        #     if i == "1":
+        #         counter += 1
+
+        # return counter
diff --git a/reverse-bits/Leo.py b/reverse-bits/Leo.py
new file mode 100644
index 000000000..3fca085cc
--- /dev/null
+++ b/reverse-bits/Leo.py
@@ -0,0 +1,16 @@
+class Solution:
+    def reverseBits(self, n: int) -> int:
+
+        res = 0
+
+        for i in range(32):
+            if n & 1:
+                res += 1 << (31 - i)
+            n >>= 1
+
+        return res
+
+        # TC: O(1), SC: O(1) for both codes
+
+        # n = bin(n)[2:].zfill(32)
+        # return int(n[::-1], 2)