diff --git a/linked-list-cycle/devyejin.py b/linked-list-cycle/devyejin.py
new file mode 100644
index 000000000..bcedbd2e0
--- /dev/null
+++ b/linked-list-cycle/devyejin.py
@@ -0,0 +1,35 @@
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, x):
+        self.val = x
+        self.next = None
+
+from typing import Optional
+
+# time complexity O(n)
+# space complexity O(1)
+class Solution:
+    def hasCycle(self, head: Optional[ListNode]) -> bool:
+        slow, fast = head, head
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+            if slow == fast:
+                return True
+        return False
+
+
+# time complexity O(n)
+# space complexity O(n)
+# class Solution:
+#     def hasCycle(self, head: Optional[ListNode]) -> bool:
+#         visited = set()
+#         while head:
+#             if head in visited:
+#                 return True
+#             visited.add(head)
+#             head = head.next
+#         return False
+
+
diff --git a/maximum-product-subarray/devyejin.py b/maximum-product-subarray/devyejin.py
new file mode 100644
index 000000000..b2034a76f
--- /dev/null
+++ b/maximum-product-subarray/devyejin.py
@@ -0,0 +1,19 @@
+from typing import List
+
+# time O(n), space O(1)
+class Solution:
+    def maxProduct(self, nums: List[int]) -> int:
+        max_prod = nums[0]
+        min_prod = nums[0]
+        result = nums[0]
+
+        for i in range(1, len(nums)):
+            candidates = (nums[i], nums[i] * max_prod, nums[i] * min_prod)
+            max_prod, min_prod = max(candidates), min(candidates)
+            result = max(result, max_prod)
+
+        return result
+
+
+
+
diff --git a/pacific-atlantic-water-flow/devyejin.py b/pacific-atlantic-water-flow/devyejin.py
new file mode 100644
index 000000000..fe8496730
--- /dev/null
+++ b/pacific-atlantic-water-flow/devyejin.py
@@ -0,0 +1,45 @@
+from typing import List
+from collections import deque
+
+# time O(mn), saoce O(mn)
+class Solution:
+    def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
+        m, n = len(heights), len(heights[0])
+        # 방문 배열
+        pacific = [[False] * n for _ in range(m)]
+        atlantic = [[False] * n for _ in range(m)]
+
+        directions = [(0, 1), (1, 0), (0, -1), (-1, 0)]
+
+        def bfs(r, c, visited):
+            queue = deque([(r, c)])
+            visited[r][c] = True
+
+            while queue:
+                r, c = queue.popleft()
+
+                for dr, dc in directions:
+                    new_r, new_c = r + dr, c + dc
+                    if (0 <= new_r < m and 0 <= new_c < n
+                            and not visited[new_r][new_c]
+                            and heights[new_r][new_c] >= heights[r][c]):
+                        visited[new_r][new_c] = True
+                        queue.append((new_r, new_c))
+
+        for i in range(n):
+            bfs(0, i, pacific)
+            bfs(m - 1, i, atlantic)
+        for i in range(m):
+            bfs(i, 0, pacific)
+            bfs(i, n - 1, atlantic)
+
+        result = []
+        for i in range(m):
+            for j in range(n):
+                if pacific[i][j] and atlantic[i][j]:
+                    result.append([i, j])
+
+        return result
+
+
+