diff --git a/3sum/Yiseull.py b/3sum/Yiseull.py
new file mode 100644
index 0000000000..340b0a09e2
--- /dev/null
+++ b/3sum/Yiseull.py
@@ -0,0 +1,27 @@
+class Solution:
+    def threeSum(self, nums: list[int]) -> list[list[int]]:
+        answer = set()
+
+        nums = sorted(nums)
+
+        n = len(nums)
+        for i in range(n - 2):
+            if nums[i] > 0:
+                break
+
+            if i > 0 and nums[i] == nums[i - 1]:
+                continue
+
+            left, right = i + 1, n - 1
+            while left < right:
+                threeSum = nums[i] + nums[left] + nums[right]
+                if threeSum < 0:
+                    left += 1
+                elif threeSum == 0:
+                    answer.add((nums[i], nums[left], nums[right]))
+                    left += 1
+                    right -= 1
+                else:
+                    right -= 1
+
+        return list(answer)
diff --git a/climbing-stairs/Yiseull.py b/climbing-stairs/Yiseull.py
new file mode 100644
index 0000000000..c124f1f6d8
--- /dev/null
+++ b/climbing-stairs/Yiseull.py
@@ -0,0 +1,9 @@
+class Solution:
+    def climbStairs(self, n: int) -> int:
+        if n <= 2: return n
+
+        p, q = 1, 2
+        for i in range(3, n + 1):
+            p, q = q, p + q
+
+        return q
diff --git a/product-of-array-except-self/Yiseull.py b/product-of-array-except-self/Yiseull.py
new file mode 100644
index 0000000000..d700e99bb8
--- /dev/null
+++ b/product-of-array-except-self/Yiseull.py
@@ -0,0 +1,15 @@
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        n = len(nums)
+        answer = [1]
+
+        # answer[i] -> nums[i] 왼쪽 값들의 곱
+        for i in range(1, n):
+            answer.append(answer[i - 1] * nums[i - 1])
+
+        tmp = 1
+        for i in reversed(range(n)):
+            answer[i] *= tmp
+            tmp *= nums[i]
+
+        return answer
diff --git a/valid-anagram/Yiseull.py b/valid-anagram/Yiseull.py
new file mode 100644
index 0000000000..2e84c297f7
--- /dev/null
+++ b/valid-anagram/Yiseull.py
@@ -0,0 +1,8 @@
+from collections import Counter
+
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        if len(s) != len(t):
+            return False
+
+        return Counter(s) == Counter(t)
diff --git a/validate-binary-search-tree/Yiseull.java b/validate-binary-search-tree/Yiseull.java
new file mode 100644
index 0000000000..40a4ce4be1
--- /dev/null
+++ b/validate-binary-search-tree/Yiseull.java
@@ -0,0 +1,27 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        return validate(root, null, null);
+    }
+
+    private boolean validate(TreeNode node, Integer low, Integer high) {
+        if (node == null) return true;
+        if (low != null && low >= node.val) return false;
+        if (high != null && high <= node.val) return false;
+        return validate(node.left, low, node.val) && validate(node.right, node.val, high);
+    }
+}