diff --git a/linked-list-cycle/ayosecu.py b/linked-list-cycle/ayosecu.py
new file mode 100644
index 000000000..18b64f4c3
--- /dev/null
+++ b/linked-list-cycle/ayosecu.py
@@ -0,0 +1,53 @@
+from typing import Optional
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, x):
+        self.val = x
+        self.next = None
+
+class Solution:
+    """
+        - Time Complexity: O(n), n = The number of nodes
+        - Space Complexity: O(1)
+    """
+    def hasCycle(self, head: Optional[ListNode]) -> bool:
+        if not head:
+            return False
+
+        slow, fast = head, head
+        while slow and fast:
+            slow = slow.next
+            if fast.next:
+                fast = fast.next.next
+            else:
+                return False
+
+            if fast == slow:
+                return True
+            
+        return False
+
+# Run Test Cases
+def do_test():
+    sol = Solution()
+
+    h1 = ListNode(3)
+    h1.next = ListNode(2)
+    h1.next.next = ListNode(0)
+    h1.next.next = ListNode(-4)
+    h1.next.next.next = h1.next
+    r1 = sol.hasCycle(h1)
+    print(f"TC 1 is Passed!" if r1 == True else f"TC 1 is Failed!")
+
+    h2 = ListNode(1)
+    h2.next = ListNode(2)
+    h2.next.next = h2
+    r2 = sol.hasCycle(h2)
+    print(f"TC 2 is Passed!" if r2 == True else f"TC 2 is Failed!")
+
+    h3 = ListNode(1)
+    r3 = sol.hasCycle(h3)
+    print(f"TC 3 is Passed!" if r3 == False else f"TC 3 is Failed!")
+
+do_test()
diff --git a/maximum-product-subarray/ayosecu.py b/maximum-product-subarray/ayosecu.py
new file mode 100644
index 000000000..b622ff676
--- /dev/null
+++ b/maximum-product-subarray/ayosecu.py
@@ -0,0 +1,30 @@
+from typing import List
+
+class Solution:
+    """
+        - Time Complexity: O(n), n = len(nums)
+        - Space Complexity: O(1)
+    """
+    def maxProduct(self, nums: List[int]) -> int:
+        max_now, max_prod, min_prod = nums[0], nums[0], nums[0]
+        
+        for num in nums[1:]:
+            if num < 0:
+                # if number is negative, swap the min/max value
+                max_prod, min_prod = min_prod, max_prod          
+
+            max_prod = max(num, max_prod * num)
+            min_prod = min(num, min_prod * num)
+            max_now = max(max_now, max_prod)          
+        
+        return max_now
+    
+tc = [
+        ([2,3,-2,4], 6),
+        ([-2,0,-1], 0)
+]
+
+sol = Solution()
+for i, (n, e) in enumerate(tc, 1):
+    r = sol.maxProduct(n)
+    print(f"TC {i} is Passed!" if r == e else f"TC {i} is Failed! - Expected: {e}, Result: {r}")
diff --git a/minimum-window-substring/ayosecu.py b/minimum-window-substring/ayosecu.py
new file mode 100644
index 000000000..b26b81e64
--- /dev/null
+++ b/minimum-window-substring/ayosecu.py
@@ -0,0 +1,47 @@
+from collections import Counter, defaultdict
+
+class Solution:
+    """
+        - Time Complexity: O(n), n = len(s)
+        - Space Complexity: O(m), m = len(t)
+    """
+    def minWindow(self, s: str, t: str) -> str:
+        need = Counter(t)
+        need_len = len(need)        
+        win_dic = defaultdict(int)
+        cnt = 0
+        min_range, min_len = [-1, -1], float("inf")
+
+        l = 0
+        for r in range(len(s)):
+            c = s[r]
+            win_dic[c] += 1
+
+            if c in need and win_dic[c] == need[c]:
+                cnt += 1
+
+            while cnt == need_len:
+                # Update min range
+                check_len = r - l + 1
+                if check_len < min_len:
+                    min_range = [l, r]
+                    min_len = check_len
+
+                # Move left pointer
+                win_dic[s[l]] -= 1
+                if s[l] in need and win_dic[s[l]] < need[s[l]]:
+                    cnt -= 1
+                l += 1
+
+        return s[min_range[0]:min_range[1]+1] if min_len != float("inf") else ""
+
+tc = [
+        ("ADOBECODEBANC", "ABC", "BANC"),
+        ("a", "a", "a"),
+        ("a", "aa", "")
+]
+
+sol = Solution()
+for i, (s, t, e) in enumerate(tc, 1):
+    r = sol.minWindow(s, t)
+    print(f"TC {i} is Passed!" if r == e else f"TC {i} is Failed! - Expected: {e}, Result: {r}")
diff --git a/pacific-atlantic-water-flow/ayosecu.py b/pacific-atlantic-water-flow/ayosecu.py
new file mode 100644
index 000000000..4060247af
--- /dev/null
+++ b/pacific-atlantic-water-flow/ayosecu.py
@@ -0,0 +1,66 @@
+from typing import List
+from collections import deque
+
+class Solution:
+    """
+        - Time Complexity: O(mn)
+        - Space Complexity: O(mn)
+    """
+    def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
+        m, n = len(heights), len(heights[0])
+
+        # Visit checking for each ocean
+        p_visited = [[False] * n for _ in range(m)]
+        a_visited = [[False] * n for _ in range(m)]
+
+        def bfs(q, visited):
+            dir = [(-1, 0), (0, -1), (1, 0), (0, 1)]
+            while q:
+                r, c = q.popleft()
+                for dx, dy in dir:
+                    next_r, next_c = r + dx, c + dy
+                    # checking next cell is not visited and higher than current cell
+                    if ( 0 <= next_r < m and 0 <= next_c < n and not visited[next_r][next_c]
+                        and heights[next_r][next_c] >= heights[r][c] ):
+                        visited[next_r][next_c] = True
+                        q.append((next_r, next_c))
+                    
+        p_dq, a_dq = deque(), deque()
+
+        for i in range(m):
+            # left side (pacific)
+            p_dq.append((i, 0))
+            p_visited[i][0] = True
+            # right side (atlantic)
+            a_dq.append((i, n - 1))
+            a_visited[i][n - 1] = True
+        
+        for j in range(n):
+            # top side (pacific)
+            p_dq.append((0, j))
+            p_visited[0][j] = True
+            # bottom side (atlantic)
+            a_dq.append((m - 1, j))
+            a_visited[m - 1][j] = True
+        
+        bfs(p_dq, p_visited)
+        bfs(a_dq, a_visited)
+
+        result = []
+        for i in range(m):
+            for j in range(n):
+                if p_visited[i][j] and a_visited[i][j]:
+                    # both of oceans are accessible
+                    result.append([i, j])
+        
+        return result
+
+tc = [
+        ([[1,2,2,3,5],[3,2,3,4,4],[2,4,5,3,1],[6,7,1,4,5],[5,1,1,2,4]], [[0,4],[1,3],[1,4],[2,2],[3,0],[3,1],[4,0]]),
+        ([[1]], [[0,0]])
+]
+
+sol = Solution()
+for i, (h, e) in enumerate(tc, 1):
+    r = sol.pacificAtlantic(h)
+    print(f"TC {i} is Passed!" if r == e else f"TC {i} is Failed! - Expected: {e}, Result: {r}")
diff --git a/sum-of-two-integers/ayosecu.py b/sum-of-two-integers/ayosecu.py
new file mode 100644
index 000000000..259d0a3d0
--- /dev/null
+++ b/sum-of-two-integers/ayosecu.py
@@ -0,0 +1,27 @@
+class Solution:
+    """
+        - Time Complexity: O(1), <= 32 bit operation
+        - Space Complexity: O(1)
+    """
+    def getSum(self, a: int, b: int) -> int:
+        # Using mask value for making 32 bit integer operation
+        MASK = 0xFFFFFFFF
+        MAX_INT = 0x7FFFFFFF
+
+        while b != 0:
+            xor = (a ^ b) & MASK
+            carry = ((a & b) << 1) & MASK
+            a, b = xor, carry
+        
+        # If a is overflow than MAX_INT (Negative Integer), return 2's compliment value
+        return a if a <= MAX_INT else ~(a ^ MASK)
+
+tc = [
+        (1, 2, 3),
+        (2, 3, 5)
+]
+
+sol = Solution()
+for i, (a, b, e) in enumerate(tc, 1):
+    r = sol.getSum(a, b)
+    print(f"TC {i} is Passed!" if r == e else f"TC {i} is Failed! - Expected: {e}, Result: {r}")