diff --git a/linked-list-cycle/yuseok89.py b/linked-list-cycle/yuseok89.py
new file mode 100644
index 0000000000..6cb4c47072
--- /dev/null
+++ b/linked-list-cycle/yuseok89.py
@@ -0,0 +1,22 @@
+# TC: O(N)
+# SC: O(1)
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution:
+    def hasCycle(self, head: Optional[ListNode]) -> bool:
+        if not head:
+            return False
+
+        slow, fast = head, head.next
+        while slow is not fast:
+            if not fast or not fast.next:
+                return False
+
+            slow, fast = slow.next, fast.next.next
+
+        return True
+
diff --git a/maximum-product-subarray/yuseok89.py b/maximum-product-subarray/yuseok89.py
new file mode 100644
index 0000000000..bfb3cbb99f
--- /dev/null
+++ b/maximum-product-subarray/yuseok89.py
@@ -0,0 +1,17 @@
+# TC: O(N)
+# SC: O(1)
+class Solution:
+    def maxProduct(self, nums: List[int]) -> int:
+
+        ans = nums[0]
+        max_val, min_val = 1, 1
+
+        for num in nums:
+            temp = max_val * num
+            max_val = max(temp, min_val * num, num)
+            min_val = min(temp, min_val * num, num)
+
+            ans = max(ans, max_val)
+
+        return ans
+
diff --git a/minimum-window-substring/yuseok89.py b/minimum-window-substring/yuseok89.py
new file mode 100644
index 0000000000..728a516eb9
--- /dev/null
+++ b/minimum-window-substring/yuseok89.py
@@ -0,0 +1,45 @@
+# TC: O(N)
+# SC: O(N)
+class Solution:
+    def minWindow(self, s: str, t: str) -> str:
+
+        len_s = len(s)
+        len_t = len(t)
+
+        min_len = len_s + 1
+        ans_start_idx = -1
+        total_cnt = 0
+
+        t_cnt_dict = Counter(t)
+        cnt_dict = defaultdict(int)
+
+        l, r = 0, 0
+
+        while r < len_s:
+
+            r_c = s[r]
+
+            if cnt_dict[r_c] < t_cnt_dict.get(r_c, 0):
+                total_cnt += 1
+
+            cnt_dict[r_c] += 1
+            r += 1
+
+            while total_cnt == len_t:
+                if min_len > r - l:
+                    min_len = r - l
+                    ans_start_idx = l
+
+                l_c = s[l]
+
+                if cnt_dict[l_c] <= t_cnt_dict[l_c]:
+                    total_cnt -= 1
+
+                cnt_dict[l_c] -= 1
+                l += 1
+
+        if ans_start_idx == -1:
+            return ''
+
+        return s[ans_start_idx:ans_start_idx + min_len]
+
diff --git a/pacific-atlantic-water-flow/yuseok89.py b/pacific-atlantic-water-flow/yuseok89.py
new file mode 100644
index 0000000000..598901b20b
--- /dev/null
+++ b/pacific-atlantic-water-flow/yuseok89.py
@@ -0,0 +1,44 @@
+# TC: O(N*M)
+# SC: O(N*M)
+class Solution:
+    def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
+
+        n = len(heights)
+        m = len(heights[0])
+
+        dirs = [[0, 1], [0, -1], [1, 0], [-1, 0]]
+
+        def dfs(row, col, check):
+            check[row][col] = True
+
+            for dir in dirs:
+                new_row = row + dir[0]
+                new_col = col + dir[1]
+
+                if 0 <= new_row < n and 0 <= new_col < m and not check[new_row][new_col] and heights[row][col] <= heights[new_row][new_col]:
+                    dfs(new_row, new_col, check)
+
+        pac_check = [[False for _ in range(m)] for _ in range(n)]
+        atl_check = [[False for _ in range(m)] for _ in range(n)]
+
+        for row in range(1, n):
+            dfs(row, 0, pac_check)
+
+        for col in range(m):
+            dfs(0, col, pac_check)
+
+        for row in range(n - 1):
+            dfs(row, m - 1, atl_check)
+
+        for col in range(m):
+            dfs(n - 1, col, atl_check)
+
+        ans = []
+
+        for row in range(n):
+            for col in range(m):
+                if pac_check[row][col] and atl_check[row][col]:
+                    ans.append([row, col])
+
+        return ans
+
diff --git a/sum-of-two-integers/yuseok89.py b/sum-of-two-integers/yuseok89.py
new file mode 100644
index 0000000000..9052b32c29
--- /dev/null
+++ b/sum-of-two-integers/yuseok89.py
@@ -0,0 +1,19 @@
+# TC: O(12)
+# SC: O(1)
+class Solution:
+    def getSum(self, a: int, b: int) -> int:
+        MASK = 0b111111111111
+
+        a = a & MASK
+        b = b & MASK
+
+        while b != 0:
+            c = (a & b) << 1
+            a = a ^ b
+            b = c & MASK
+
+        if a > (MASK >> 1):
+            a = ~(a ^ MASK)
+
+        return a
+