diff --git a/combination-sum/yuseok89.py b/combination-sum/yuseok89.py
new file mode 100644
index 000000000..6003c0fc2
--- /dev/null
+++ b/combination-sum/yuseok89.py
@@ -0,0 +1,28 @@
+# TC: O(N^T)
+# SC: O(T)
+class Solution:
+
+    def rec(self, cur_idx, target, candi, cur_combi, ans):
+
+        if target == 0:
+            ans.append(cur_combi.copy())
+
+        if cur_idx == len(candi):
+            return
+
+        for idx in range(cur_idx, len(candi)):
+            if target < candi[idx]:
+                continue
+
+            cur_combi.append(candi[idx])
+            self.rec(idx, target - candi[idx], candi, cur_combi, ans)
+            cur_combi.pop()
+
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+
+        ans = []
+
+        self.rec(0, target, candidates, [], ans)
+
+        return ans
+
diff --git a/decode-ways/yuseok89.py b/decode-ways/yuseok89.py
new file mode 100644
index 000000000..da6c0da32
--- /dev/null
+++ b/decode-ways/yuseok89.py
@@ -0,0 +1,29 @@
+# TC: O(N)
+# SC: O(N)
+class Solution:
+
+    memo = []
+
+    def rec(self, idx, s, n):
+        if self.memo[idx] == -1:
+
+            cnt = 0
+
+            if int(s[idx]) != 0:
+                cnt += self.rec(idx + 1, s, n)
+
+            if idx + 1 < n:
+                if int(s[idx]) != 0 and int(s[idx]) * 10 + int(s[idx + 1]) <= 26:
+                    cnt += self.rec(idx + 2, s, n)
+
+            self.memo[idx] = cnt
+
+        return self.memo[idx]
+
+    def numDecodings(self, s: str) -> int:
+
+        self.memo = [-1] * (len(s) + 1)
+        self.memo[len(s)] = 1
+
+        return self.rec(0, s, len(s))
+
diff --git a/maximum-subarray/yuseok89.py b/maximum-subarray/yuseok89.py
new file mode 100644
index 000000000..0056aae9b
--- /dev/null
+++ b/maximum-subarray/yuseok89.py
@@ -0,0 +1,24 @@
+# TC: O(N)
+# SC: O(1)
+class Solution:
+    def maxSubArray(self, nums: List[int]) -> int:
+        sum = nums[0]
+        ans = sum
+
+        for num in nums[1:]:
+            sum = max(num, sum + num)
+            ans = max(ans, sum)
+
+        return ans
+
+# class Solution:
+#     def maxSubArray(self, nums: List[int]) -> int:
+#         sum = nums[0] if nums[0] < 0 else 0
+#         ans = sum
+
+#         for num in nums:
+#             sum = max(num, sum + num)
+#             ans = max(ans, sum)
+
+#         return ans
+
diff --git a/number-of-1-bits/yuseok89.py b/number-of-1-bits/yuseok89.py
new file mode 100644
index 000000000..a70faf428
--- /dev/null
+++ b/number-of-1-bits/yuseok89.py
@@ -0,0 +1,13 @@
+# TC: O(logN)
+# SC: O(1)
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+
+        cnt = 0
+
+        while n > 0:
+            cnt = cnt + (n % 2)
+            n //= 2
+
+        return cnt
+
diff --git a/valid-palindrome/yuseok89.py b/valid-palindrome/yuseok89.py
new file mode 100644
index 000000000..4af870bc8
--- /dev/null
+++ b/valid-palindrome/yuseok89.py
@@ -0,0 +1,43 @@
+# TC: O(N)
+# SC: O(1)
+class Solution:
+
+    def is_alphanum(self, c):
+        return 'a' <= c <= 'z' or '0' <= c <= '9'
+
+    def isPalindrome(self, s: str) -> bool:
+
+        left = 0
+        right = len(s) - 1
+
+        s = s.lower()
+
+        while(left < right):
+            while left < len(s) and not self.is_alphanum(s[left]):
+                left += 1
+            while right >= 0 and not self.is_alphanum(s[right]):
+                right -= 1
+
+            if left < right and s[left] != s[right]:
+                return False
+
+            left += 1
+            right -= 1
+
+        return True
+
+# class Solution:
+#     def isPalindrome(self, s: str) -> bool:
+#
+#         import re
+#
+#         s = re.sub(r'[^a-zA-Z0-9]', '', s.lower())
+#
+#         n = len(s)
+#
+#         for idx in range(0, n // 2):
+#             if s[idx] != s[n - idx - 1]:
+#                 return False
+#
+#         return True
+