diff --git a/best-time-to-buy-and-sell-stock/Leo.py b/best-time-to-buy-and-sell-stock/Leo.py
new file mode 100644
index 000000000..783de27da
--- /dev/null
+++ b/best-time-to-buy-and-sell-stock/Leo.py
@@ -0,0 +1,11 @@
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        minPrice = prices[0]
+        maxPro = 0
+
+        for i in range(1, len(prices)):
+            maxPro = max(maxPro, prices[i] - minPrice)
+            minPrice = min (minPrice, prices[i])
+
+        return maxPro
+        ## TC: O(n) SC: O(1)..?
diff --git a/contains-duplicate/Leo.py b/contains-duplicate/Leo.py
new file mode 100644
index 000000000..abeaf421b
--- /dev/null
+++ b/contains-duplicate/Leo.py
@@ -0,0 +1,13 @@
+class Solution:
+    def containsDuplicate(self, nums: List[int]) -> bool:
+        return len(set(nums)) != len(nums)
+
+        # seen = set()
+
+        # for i in nums:   ## O(n)
+        #     if i in seen: ## O(1)
+        #         return True
+        #     else:
+        #         seen.add(i)
+
+        # return False
diff --git a/two-sum/Leo.py b/two-sum/Leo.py
new file mode 100644
index 000000000..37686a1d3
--- /dev/null
+++ b/two-sum/Leo.py
@@ -0,0 +1,13 @@
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        seen = {}
+
+        for key, val in enumerate(nums):
+            diff = target - val
+
+            if diff in seen:
+                return [seen[diff], key]
+            else:
+                seen[val] = key
+
+    # TC: O(n), SC: O(n)
diff --git a/valid-anagram/Leo.py b/valid-anagram/Leo.py
new file mode 100644
index 000000000..d5081a794
--- /dev/null
+++ b/valid-anagram/Leo.py
@@ -0,0 +1,18 @@
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+
+        counter_dict = collections.defaultdict(int)
+
+        for i in s:
+            counter_dict[i] += 1
+
+        for i in t:
+            counter_dict[i] -= 1
+
+        for val in counter_dict.values():
+            if val != 0:
+                return False
+
+        return True
+        # TC:O(n), SC: O(len(s or t))
+        # return sorted(s) == sorted(t) ## O(nlogn)
diff --git a/valid-palindrome/Leo.py b/valid-palindrome/Leo.py
new file mode 100644
index 000000000..291113c22
--- /dev/null
+++ b/valid-palindrome/Leo.py
@@ -0,0 +1,7 @@
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
+        ans = [i for i in s.lower() if i.isalnum()]
+        return ans == ans[::-1]
+
+        # TC: O(n)
+        # SC: O(n)