diff --git a/binary-tree-level-order-traversal/Leo.py b/binary-tree-level-order-traversal/Leo.py
new file mode 100644
index 000000000..9c12cb828
--- /dev/null
+++ b/binary-tree-level-order-traversal/Leo.py
@@ -0,0 +1,25 @@
+class Solution:
+    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        if root is None:
+            return []
+
+        queue = deque([root])
+        return_list = []
+
+        while queue:
+            level_size = len(queue)
+            current_level = []
+
+            for n in range(level_size):
+                node = queue.popleft()
+                current_level.append(node.val)
+                if node.left:
+                    queue.append(node.left)
+                if node.right:
+                    queue.append(node.right)
+
+            return_list.append(current_level)
+
+        return return_list
+
+        ## TC: O(n), SC: O(n)
diff --git a/lowest-common-ancestor-of-a-binary-search-tree/Leo.py b/lowest-common-ancestor-of-a-binary-search-tree/Leo.py
new file mode 100644
index 000000000..cacd5fa46
--- /dev/null
+++ b/lowest-common-ancestor-of-a-binary-search-tree/Leo.py
@@ -0,0 +1,14 @@
+class Solution:
+    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
+
+        while True:
+            if root.val < min(p.val, q.val):
+                root = root.right
+
+            elif root.val > max(p.val, q.val):
+                root = root.left
+
+            else:
+                return root
+
+        # TC: O(n) where n is height of the tree, SC: O(1)
diff --git a/remove-nth-node-from-end-of-list/Leo.py b/remove-nth-node-from-end-of-list/Leo.py
new file mode 100644
index 000000000..967be6cbc
--- /dev/null
+++ b/remove-nth-node-from-end-of-list/Leo.py
@@ -0,0 +1,19 @@
+class Solution:
+    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
+
+        left = right = head
+
+        for i in range(n):
+            right = right.next
+
+        if not right: return head.next
+
+        while right.next:
+            left = left.next
+            right = right.next
+
+        left.next = left.next.next  ## deleting node
+
+        return head
+
+        # TC: O(n), SC: O(1)
diff --git a/reorder-list/Leo.py b/reorder-list/Leo.py
new file mode 100644
index 000000000..0f74c91ef
--- /dev/null
+++ b/reorder-list/Leo.py
@@ -0,0 +1,23 @@
+class Solution:
+    def reorderList(self, head: Optional[ListNode]) -> None:
+        """
+        Do not return anything, modify head in-place instead.
+        """
+        if not head:
+            return
+
+        slow, fast = head, head
+        while fast and fast.next:
+            slow, fast = slow.next, fast.next.next
+
+        prev, curr = None, slow
+
+        while curr:
+            curr.next, prev, curr = prev, curr, curr.next
+
+        first, second = head, prev
+        while second.next:
+            first.next, first = second, first.next
+            second.next, second = first, second.next
+
+        ## TC: O(n), SC: O(1)
diff --git a/validate-binary-search-tree/Leo.py b/validate-binary-search-tree/Leo.py
new file mode 100644
index 000000000..63658cfc1
--- /dev/null
+++ b/validate-binary-search-tree/Leo.py
@@ -0,0 +1,12 @@
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def helper(node, low, high):
+            if not node:
+                return True
+            if not (low < node.val < high):
+                return False
+            return helper(node.left, low, node.val) and helper(node.right, node.val, high)
+
+        return helper(root, -inf, inf)
+
+        ## TC: O(n), SC: O(n)