diff --git a/3sum/jiyseo.py b/3sum/jiyseo.py
new file mode 100644
index 000000000..779f97b2b
--- /dev/null
+++ b/3sum/jiyseo.py
@@ -0,0 +1,43 @@
+class Solution(object):
+    def threeSum(self, nums):
+        # 시간복잡도 = O(N^3)
+        # length = len(nums)
+        # result = []
+        # for i in range(length - 1) :
+        #     for j in range(i + 1, length) :
+        #         s = - (nums[i] + nums[j])
+        #         if s in nums[j + 1 : length] :
+        #             result.append(sorted((nums[i], nums[j], s)))
+        
+        # return(list(set(map(tuple, result))))
+
+        # 시간 복잡도 = O(N^2)
+        nums.sort()
+        length = len(nums)
+        result = []
+
+        for i in range(length - 2) :
+            if i > 0 and nums[i - 1] == nums[i] : # 같은 숫자인 경우 패스
+                continue
+            target = nums[i]
+            left = i + 1
+            right = length - 1
+
+            while left < right :
+                sum = target + nums[left] + nums[right]
+                if sum > 0 :
+                    right -= 1
+                elif sum < 0 : 
+                    left += 1
+                else :
+                    result.append([target, nums[left], nums[right]])
+                    # 중복 숫자 건너뛰기
+                    while left < right and nums[right] == nums[right - 1]:
+                        right -= 1
+                    while left < right and nums[left] == nums[left + 1]:
+                        left += 1
+                    right -= 1
+                    left += 1
+                    
+        return result
+
diff --git a/climbing-stairs/jiyseo.py b/climbing-stairs/jiyseo.py
new file mode 100644
index 000000000..be77336d0
--- /dev/null
+++ b/climbing-stairs/jiyseo.py
@@ -0,0 +1,15 @@
+class Solution(object):
+    def climbStairs(self, n):
+        # 시간복잡도 = O(N)
+        if n < 2 :
+            return n
+
+        arr = [0] * (n + 1)
+        arr[1] = 1
+        arr[2] = 2
+
+        for i in range(3, n + 1) :
+            arr[i] = arr[i - 1] + arr[i - 2]
+
+        return arr[n]
+        
diff --git a/product-of-array-except-self/jiyseo.py b/product-of-array-except-self/jiyseo.py
new file mode 100644
index 000000000..cfa50541c
--- /dev/null
+++ b/product-of-array-except-self/jiyseo.py
@@ -0,0 +1,18 @@
+class Solution(object):
+    def productExceptSelf(self, nums):
+        # 시간복잡도 = O(N)
+        n = 1
+        arr = []
+        cnt = nums.count(0)
+        for i in nums : # 0을 제외한 값들의 곱
+            if i != 0 :
+                n *= i
+        for i in nums :
+            if i == 0 and cnt == 1: # nums에 0이 한개인 경우 나머지의 곱
+                arr.append(n)
+            elif cnt >= 1 : # nums에 0이 여러개인 경우 무조건 0
+                arr.append(0)
+            else : # 그 외의 경우
+                arr.append(n // i)
+        return(arr)
+
diff --git a/valid-anagram/jiyseo.py b/valid-anagram/jiyseo.py
new file mode 100644
index 000000000..fe3ebb32a
--- /dev/null
+++ b/valid-anagram/jiyseo.py
@@ -0,0 +1,12 @@
+class Solution(object):
+    def isAnagram(self, s, t):
+        # 시간복잡도 = O(N * (N + M))
+        if len(s) != len(t) :
+            return False
+
+        for i in set(s) :
+            if s.count(i) != t.count(i) :
+                return False
+
+        return True
+
diff --git a/validate-binary-search-tree/jiyseo.py b/validate-binary-search-tree/jiyseo.py
new file mode 100644
index 000000000..c5583f3ed
--- /dev/null
+++ b/validate-binary-search-tree/jiyseo.py
@@ -0,0 +1,18 @@
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution(object):
+    # 시간복잡도 = O(V + E)
+    def isValidBST(self, root):
+        def dfs(node, low, high) :
+            if not node :
+                return True
+            if not (low < node.val < high) :
+                return False
+            return dfs(node.left, low, node.val) and dfs(node.right, node.val, high)
+
+        return dfs(root, float("-inf"), float("inf"))
+