diff --git a/combination-sum/yayyz.py b/combination-sum/yayyz.py
new file mode 100644
index 000000000..a123d786a
--- /dev/null
+++ b/combination-sum/yayyz.py
@@ -0,0 +1,16 @@
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        result = []
+
+        def dfs(path, total, start):
+            if total == target:
+                result.append(path[:])
+                return
+            if total > target:
+                return
+
+            for i in range(start, len(candidates)):
+                dfs(path + [candidates[i]], total + candidates[i], i)
+
+        dfs([], 0, 0)
+        return result
diff --git a/decode-ways/yayyz.py b/decode-ways/yayyz.py
new file mode 100644
index 000000000..9bdfb3390
--- /dev/null
+++ b/decode-ways/yayyz.py
@@ -0,0 +1,13 @@
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        n = len(s)
+        dp = [0] * (n + 1)
+        dp[n] = 1  # 빈 문자열 처리
+
+        for i in range(n - 1, -1, -1):
+            if s[i] != "0":
+                dp[i] += dp[i + 1]
+                if i + 1 < n and int(s[i:i+2]) <= 26:
+                    dp[i] += dp[i + 2]
+
+        return dp[0]
diff --git a/maximum-subarray/yayyz.py b/maximum-subarray/yayyz.py
new file mode 100644
index 000000000..0648a302e
--- /dev/null
+++ b/maximum-subarray/yayyz.py
@@ -0,0 +1,7 @@
+class Solution:
+    def maxSubArray(self, nums: List[int]) -> int:
+        curr_sum = max_sum = nums[0]
+        for num in nums[1:]:
+            curr_sum = max(num, curr_sum + num)
+            max_sum = max(max_sum, curr_sum)
+        return max_sum
diff --git a/number-of-1-bits/yayyz.py b/number-of-1-bits/yayyz.py
new file mode 100644
index 000000000..d2ddce0c1
--- /dev/null
+++ b/number-of-1-bits/yayyz.py
@@ -0,0 +1,12 @@
+# bit operation 
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        count = 0
+        while n:
+            count += n & 1
+            n >>= 1
+        return count 
+    
+# class Solution:
+#     def hammingWeight(self, n: int) -> int:
+#         return bin(n).count('1')
diff --git a/valid-palindrome/yayyz.py b/valid-palindrome/yayyz.py
new file mode 100644
index 000000000..8d4537da5
--- /dev/null
+++ b/valid-palindrome/yayyz.py
@@ -0,0 +1,15 @@
+class Solution:
+    def isPalindrome(self, s: str) -> bool:        
+        s = ''.join(filter(str.isalnum, s)).lower()
+        if not (s and s.strip()): return True 
+        
+        head = 0
+        tail = len(s) -1
+        while head < tail:
+            if s[head] != s[tail]:
+                return False
+            else: 
+                head += 1
+                tail -= 1
+        
+        return True        
diff --git a/validate-binary-search-tree/yayyz.py b/validate-binary-search-tree/yayyz.py
new file mode 100644
index 000000000..8764aa65d
--- /dev/null
+++ b/validate-binary-search-tree/yayyz.py
@@ -0,0 +1,25 @@
+# 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 isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def is_bst(node):
+            if not node:
+                return True, float('inf'), float('-inf')  # (is_bst, min_val, max_val)
+
+            left_valid, left_min, left_max = is_bst(node.left)
+            right_valid, right_min, right_max = is_bst(node.right)
+
+            if not left_valid or not right_valid:
+                return False, 0, 0
+
+            if not (left_max < node.val < right_min):
+                return False, 0, 0
+
+            return True, min(left_min, node.val), max(right_max, node.val)
+
+        valid, _, _ = is_bst(root)
+        return valid