diff --git a/linked-list-cycle/hu6r1s.py b/linked-list-cycle/hu6r1s.py
new file mode 100644
index 000000000..c6603da7a
--- /dev/null
+++ b/linked-list-cycle/hu6r1s.py
@@ -0,0 +1,18 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution:
+  """
+  생각보다 쉬운 문제.
+  """
+  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/hu6r1s.py b/maximum-product-subarray/hu6r1s.py
new file mode 100644
index 000000000..28de63cf9
--- /dev/null
+++ b/maximum-product-subarray/hu6r1s.py
@@ -0,0 +1,8 @@
+class Solution:
+    def maxProduct(self, nums: List[int]) -> int:
+        result = nums[0]
+        min_prod = max_prod = 1
+        for num in nums:
+            min_prod, max_prod = min(min_prod * num, max_prod * num, num), max(min_prod * num, max_prod * num, num)
+            result = max(max_prod, result)
+        return result
diff --git a/minimum-window-substring/hu6r1s.py b/minimum-window-substring/hu6r1s.py
new file mode 100644
index 000000000..0f7155f5e
--- /dev/null
+++ b/minimum-window-substring/hu6r1s.py
@@ -0,0 +1,44 @@
+class Solution:
+    """
+    접근 방법: https://www.youtube.com/watch?v=jSto0O4AJbM
+        - t의 문자 개수를 미리 세어둔 후(t_count),
+          s에서 두 포인터(left, right)를 이용해 슬라이딩 윈도우를 확장/축소하며
+          모든 조건을 만족하는 최소 구간을 찾는다.
+        - have: 현재 window에서 조건을 만족한 문자 종류 개수
+        - need: t에 있는 문자 종류 개수
+        - 조건을 만족할 때마다 왼쪽 포인터를 줄여 최소 길이 갱신
+
+        시간복잡도:
+        - 각 문자를 오른쪽 포인터로 한 번, 왼쪽 포인터로 한 번만 방문 → O(|s|)
+        - t의 해시맵 구성 O(|t|)
+        - 최종 시간복잡도 = O(|s| + |t|)
+
+        공간복잡도:
+        - window와 t_count 해시맵 → O(|s| + |t|)
+    """
+    def minWindow(self, s: str, t: str) -> str:
+        t_count, window = {}, {}
+
+        for c in t:
+            t_count[c] = 1 + t_count.get(c, 0)
+        
+        have, need = 0, len(t_count)
+        res, len_res = [-1, -1], float("infinity")
+        left = 0
+        for right in range(len(s)):
+            c = s[right]
+            window[c] = 1 + window.get(c, 0)
+
+            if c in t_count and window[c] == t_count[c]:
+                have += 1
+            
+            while have == need:
+                if (right - left + 1) < len_res:
+                    res = [left, right]
+                    len_res = (right - left + 1)
+                window[s[left]] -= 1
+                if s[left] in t_count and window[s[left]] < t_count[s[left]]:
+                    have -= 1
+                left += 1
+        left, right = res
+        return s[left:right+1] if len_res != float("infinity") else ""
diff --git a/pacific-atlantic-water-flow/hu6r1s.py b/pacific-atlantic-water-flow/hu6r1s.py
new file mode 100644
index 000000000..e0904c690
--- /dev/null
+++ b/pacific-atlantic-water-flow/hu6r1s.py
@@ -0,0 +1,33 @@
+class Solution:
+    """
+    https://www.youtube.com/watch?v=s-VkcjHqkGI
+    그래프 문제여서 바로 dfs나 bfs로 불면 될 것 같다는 생각이 들었는데 어떻게 해야할지 막막했다. 영상의 방식을 보고 공부
+    """
+    def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
+        rows, cols = len(heights), len(heights[0])
+        pac, atl = set(), set()
+
+        def dfs(r, c, visit, prevHeight):
+            if ((r, c) in visit or r < 0 or c < 0 or r == rows or c == cols or heights[r][c] < prevHeight):
+                return
+            visit.add((r, c))
+            dfs(r + 1, c, visit, heights[r][c])
+            dfs(r - 1, c, visit, heights[r][c])
+            dfs(r, c + 1, visit, heights[r][c])
+            dfs(r, c - 1, visit, heights[r][c])
+
+        
+        for c in range(cols):
+            dfs(0, c, pac, heights[0][c])
+            dfs(rows - 1, c, atl, heights[rows-1][c])
+        
+        for r in range(rows):
+            dfs(r, 0, pac, heights[r][0])
+            dfs(r, cols - 1, atl, heights[r][cols - 1])
+
+        result = []
+        for r in range(rows):
+            for c in range(cols):
+                if (r, c) in pac and (r, c) in atl:
+                    result.append([r, c])
+        return results
diff --git a/sum-of-two-integers/hu6r1s.py b/sum-of-two-integers/hu6r1s.py
new file mode 100644
index 000000000..796fbcd66
--- /dev/null
+++ b/sum-of-two-integers/hu6r1s.py
@@ -0,0 +1,6 @@
+class Solution:
+    def getSum(self, a: int, b: int) -> int:
+        mask = 0xFFFFFFFF
+        while b & mask:
+            a, b = a ^ b, (a & b) << 1
+        return (a & mask) if b > 0 else a