diff --git a/3sum/sejineer.py b/3sum/sejineer.py
new file mode 100644
index 000000000..21c048506
--- /dev/null
+++ b/3sum/sejineer.py
@@ -0,0 +1,24 @@
+"""
+시간 복잡도: O(N^2)
+공간 복잡도: O(N)
+"""
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        nums.sort()
+
+        result = set()
+
+        for i in range(len(nums) - 2):
+            start, end = i + 1, len(nums) - 1
+            while start < end:
+                temp = nums[i] + nums[start] + nums[end]
+                if temp == 0:
+                    result.add((nums[i], nums[start], nums[end]))
+                    start += 1
+                    end -= 1
+                elif temp < 0:
+                    start += 1
+                else:
+                    end -= 1
+
+        return list(result)
diff --git a/climbing-stairs/sejineer.py b/climbing-stairs/sejineer.py
new file mode 100644
index 000000000..8f264a9df
--- /dev/null
+++ b/climbing-stairs/sejineer.py
@@ -0,0 +1,13 @@
+"""
+시간 복잡도: O(N)
+공간 복잡도: O(N)
+"""
+class Solution:
+    def climbStairs(self, n: int) -> int:
+        dp = [0] * 46
+        dp[1], dp[2] = 1, 2
+
+        for i in range(3, n + 1):
+            dp[i] = dp[i - 1] + dp[i - 2]
+        
+        return dp[n]
diff --git a/product-of-array-except-self/sejineer.py b/product-of-array-except-self/sejineer.py
new file mode 100644
index 000000000..ce30b10e4
--- /dev/null
+++ b/product-of-array-except-self/sejineer.py
@@ -0,0 +1,19 @@
+"""
+시간 복잡도: O(N)
+공간 복잡도: O(N)
+"""
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        prefix = [1] * len(nums)
+        for i in range(len(nums) - 1):
+            prefix[i + 1] = prefix[i] * nums[i]
+        
+        suffix = [1] * len(nums)
+        for i in range(len(nums) - 1, 0, -1):
+            suffix[i - 1] = suffix[i] * nums[i]
+        
+        result = []
+        for i, j in zip(prefix, suffix):
+            result.append(i * j)
+        
+        return result
diff --git a/valid-anagram/sejineer.py b/valid-anagram/sejineer.py
new file mode 100644
index 000000000..22bb2fc04
--- /dev/null
+++ b/valid-anagram/sejineer.py
@@ -0,0 +1,26 @@
+"""
+시간 복잡도: O(N)
+공간 복잡도: O(N)
+
+코드 가독성 개선 코드:
+from collections import Counter
+
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        return Counter(s) == Counter(t)
+"""
+from collections import Counter
+
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        if len(s) != len(t):
+            return False
+            
+        s_counter = Counter(s)
+        t_counter = Counter(t)
+
+        for num, freq in t_counter.items():
+            if s_counter[num] != freq:
+                return False
+        
+        return True
diff --git a/validate-binary-search-tree/sejineer.py b/validate-binary-search-tree/sejineer.py
new file mode 100644
index 000000000..0eed6a8c5
--- /dev/null
+++ b/validate-binary-search-tree/sejineer.py
@@ -0,0 +1,24 @@
+"""
+시간 복잡도: O(N)
+공간 복잡도: O(N)
+"""
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        num_list = []
+
+        def dfs(node: TreeNode):
+            if not node:
+                return
+            
+            dfs(node.left)
+            num_list.append(node.val)
+            dfs(node.right)
+        
+        dfs(root)
+
+        mem = num_list[0]
+        for i in range(1, len(num_list)):
+            if mem >= num_list[i]:
+                return False
+            mem = num_list[i]
+        return True