diff --git a/3sum/sangbeenmoon.py b/3sum/sangbeenmoon.py
index 8181cfdf1..e82e32c0b 100644
--- a/3sum/sangbeenmoon.py
+++ b/3sum/sangbeenmoon.py
@@ -30,3 +30,36 @@ def threeSum(self, nums: list[int]) -> list[list[int]]:
                     left = left + 1
                     right = right - 1
         return answer
+
+
+# TC : O(n^2)
+# SC : O(n)
+
+class Solution:
+    def threeSum(self, nums: list[int]) -> list[list[int]]:
+        
+        nums.sort()
+
+        answer_map = {}
+        answer = []
+        
+        for i in range(len(nums)):
+            target = -1 * nums[i]
+
+            left = i + 1
+            right = len(nums) - 1
+
+            while left < right:
+                if nums[left] + nums[right] == target:
+                    candidate = (target * -1, nums[left], nums[right])
+                    if candidate not in answer_map:
+                        answer_map[candidate] = True
+                        answer.append([target * -1, nums[left], nums[right]])
+                    left += 1
+                    right -= 1
+                elif nums[left] + nums[right] < target:
+                    left += 1
+                else:
+                    right -= 1
+        return answer
+                
diff --git a/validate-binary-search-tree/sangbeenmoon.py b/validate-binary-search-tree/sangbeenmoon.py
index c29810b48..8f8faa4a7 100644
--- a/validate-binary-search-tree/sangbeenmoon.py
+++ b/validate-binary-search-tree/sangbeenmoon.py
@@ -38,3 +38,38 @@ def go_right(self, cur:TreeNode, mm:int, MM:int):
         if cur.right:
             self.go_right(cur.right, cur.val, MM)
 
+
+
+
+# ---------
+
+
+# Definition for a binary tree node.
+
+# TC: O(n)
+# SC: O(h)
+
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        
+        def isValid(min_val, max_val, cur: TreeNode) -> bool:
+            
+
+            if cur is None:
+                return True
+            
+            if min_val is not None and cur.val <= min_val:
+                return False
+             
+            if max_val is not None and cur.val >= max_val:
+                return False
+            
+            return isValid(min_val, cur.val, cur.left) and isValid(cur.val, max_val, cur.right)
+        
+        return isValid(None, None ,root)
+