diff --git a/3sum/yuseok89.py b/3sum/yuseok89.py
new file mode 100644
index 0000000000..52d477339f
--- /dev/null
+++ b/3sum/yuseok89.py
@@ -0,0 +1,33 @@
+# TC: O(N^2)
+# SC: O(N)
+class Solution:
+    def threeSum(self, nums: list[int]) -> list[list[int]]:
+
+        cnt_dict = Counter(nums)
+        nums_uniq = sorted(cnt_dict.keys())
+        n = len(nums_uniq)
+
+        ret = []
+
+        for num in cnt_dict.keys():
+            if num == 0:
+                if cnt_dict[num] >= 3:
+                    ret.append([0, 0, 0])
+            elif cnt_dict[num] >= 2 and num * 2 * -1 in cnt_dict:
+                ret.append([num, num, num * 2 * -1])
+
+        for idx1, num1 in enumerate(nums_uniq):
+            if num1 > 0:
+                break
+
+            for idx2 in range(idx1 + 1, n):
+                num2 = nums_uniq[idx2]
+                num3 = (num1 + num2) * -1
+
+                if num2 >= num3:
+                    break
+                elif num3 in cnt_dict:
+                    ret.append([num1, num2, num3])
+
+        return ret
+
diff --git a/climbing-stairs/yuseok89.py b/climbing-stairs/yuseok89.py
new file mode 100644
index 0000000000..98908f9b6d
--- /dev/null
+++ b/climbing-stairs/yuseok89.py
@@ -0,0 +1,15 @@
+# TC: O(N)
+# SC: O(1)
+class Solution:
+    def climbStairs(self, n: int) -> int:
+
+        one_down = 1
+        two_down = 0
+
+        for _ in range(1, n):
+            cur = two_down + one_down
+            two_down = one_down
+            one_down = cur
+
+        return two_down + one_down
+
diff --git a/product-of-array-except-self/yuseok89.py b/product-of-array-except-self/yuseok89.py
new file mode 100644
index 0000000000..2acceaf3c5
--- /dev/null
+++ b/product-of-array-except-self/yuseok89.py
@@ -0,0 +1,25 @@
+# TC: O(N)
+# SC: O(N)
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+
+        n = len(nums)
+        prefix_prod = [0] * n
+        suffix_prod = [0] * n
+
+        prefix_prod[0] = nums[0]
+        suffix_prod[-1] = nums[-1]
+
+        for idx in range(1, n):
+            prefix_prod[idx] = prefix_prod[idx - 1] * nums[idx]
+            suffix_prod[-idx - 1] = suffix_prod[-idx] * nums[-idx - 1]
+
+        ret = []
+
+        ret.append(suffix_prod[1])
+        for idx in range(1, n - 1):
+            ret.append(prefix_prod[idx - 1] * suffix_prod[idx + 1])
+        ret.append(prefix_prod[n - 2])
+
+        return ret
+
diff --git a/valid-anagram/yuseok89.py b/valid-anagram/yuseok89.py
new file mode 100644
index 0000000000..f8793092e7
--- /dev/null
+++ b/valid-anagram/yuseok89.py
@@ -0,0 +1,13 @@
+# TC: O(N)
+# SC: O(1)
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+
+        if len(s) != len(t):
+            return False
+
+        cnt_s = Counter(s)
+        cnt_t = Counter(t)
+
+        return cnt_s == cnt_t
+
diff --git a/validate-binary-search-tree/yuseok89.py b/validate-binary-search-tree/yuseok89.py
new file mode 100644
index 0000000000..0e9cc2015d
--- /dev/null
+++ b/validate-binary-search-tree/yuseok89.py
@@ -0,0 +1,23 @@
+# TC: O(N)
+# SC: O(H)
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+
+    def isValid(self, left_bound, right_bound, root):
+        if root is None:
+            return True
+        if left_bound is not None and left_bound >= root.val:
+            return False
+        if right_bound is not None and right_bound <= root.val:
+            return False
+
+        return self.isValid(left_bound, root.val, root.left) and self.isValid(root.val, right_bound, root.right)
+
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        return self.isValid(None, root.val, root.left) and self.isValid(root.val, None, root.right)
+