diff --git a/longest-substring-without-repeating-characters/dusunax.py b/longest-substring-without-repeating-characters/dusunax.py
new file mode 100644
index 000000000..d11cbd638
--- /dev/null
+++ b/longest-substring-without-repeating-characters/dusunax.py
@@ -0,0 +1,63 @@
+'''
+# 3. Longest Substring Without Repeating Characters
+
+use a set to store the characters in the current substring.
+
+
+## Time and Space Complexity
+
+### Solution 1: using set
+
+```
+TC: O(n)
+SC: O(n)
+```
+
+####  TC is O(n):
+- iterating with end pointer through the string once. = O(n)
+- inner while loop runs at most once for each character in the string. = Amortized O(1)
+
+#### SC is O(n):
+- using a set to store the characters in the current substring. = O(n)
+
+### Solution 2: using ASCII array
+
+```
+TC: O(n)
+SC: O(128)
+```
+
+#### TC is O(n):
+- iterating with end pointer through the string once. = O(n)
+- checking if the character is in the current substring.
+
+#### SC is O(1):
+- using an array to store the characters in the current substring. = constant space O(128)
+'''
+class Solution:
+    def lengthOfLongestSubstringWithSet(self, s: str) -> int:
+        max_count = 0
+        start = 0
+        substrings = set() # SC: O(n)
+
+        for end in range(len(s)): # TC: O(n)
+            while s[end] in substrings: # TC: Amortized O(1)
+                substrings.remove(s[start])
+                start += 1
+            substrings.add(s[end])
+            max_count = max(max_count, end - start + 1)
+        
+        return max_count
+    
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        max_count = 0
+        start = 0
+        char_index = [-1] * 128 # SC: O(128)
+
+        for end in range(len(s)): # TC: O(n)
+            if char_index[ord(s[end])] >= start:
+                start = char_index[ord(s[end])] + 1
+            char_index[ord(s[end])] = end
+            max_count = max(max_count, end - start + 1)
+            
+        return max_count
diff --git a/number-of-islands/dusunax.py b/number-of-islands/dusunax.py
new file mode 100644
index 000000000..6a8d80ede
--- /dev/null
+++ b/number-of-islands/dusunax.py
@@ -0,0 +1,38 @@
+'''
+# 200. Number of Islands
+
+use DFS to find the number of islands (to find the all the possible cases)
+
+## Time and Space Complexity
+
+```
+TC: O(m * n)
+SC: O(m * n)
+```
+
+### TC is O(m * n):
+- dfs function is called for each cell in the grid for checking is the land ("1") = O(m * n)
+
+### SC is O(m * n):
+- using a recursive function, the call stack can go as deep as the number of cells in the grid in the worst case. = O(m * n)
+'''
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        def dfs(x, y):
+            if x < 0 or y < 0 or y >= len(grid) or x >= len(grid[0]) or grid[y][x] == "0" :
+                return
+        
+            grid[y][x] = "0"
+            dfs(x, y + 1)
+            dfs(x - 1, y)
+            dfs(x, y - 1)
+            dfs(x + 1, y)
+        
+        island_count = 0
+        for y in range(len(grid)):
+            for x in range(len(grid[0])):
+                if grid[y][x] == "1":
+                    dfs(x, y)
+                    island_count += 1
+
+        return island_count
diff --git a/reverse-linked-list/dusunax.py b/reverse-linked-list/dusunax.py
new file mode 100644
index 000000000..569cde36c
--- /dev/null
+++ b/reverse-linked-list/dusunax.py
@@ -0,0 +1,24 @@
+'''
+# 206. Reverse Linked List
+
+iterate through the linked list and reverse the direction of the pointers.
+
+## Time and Space Complexity
+
+```
+TC: O(n)
+SC: O(1)
+```
+'''
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        prev = None
+        current = head
+
+        while current is not None: # TC: O(n)
+            next_list_temp = current.next
+            current.next = prev
+            prev = current
+            current = next_list_temp
+
+        return prev
diff --git a/set-matrix-zeroes/dusunax.py b/set-matrix-zeroes/dusunax.py
new file mode 100644
index 000000000..b43766109
--- /dev/null
+++ b/set-matrix-zeroes/dusunax.py
@@ -0,0 +1,67 @@
+'''
+# 73. Set Matrix Zeroes
+# solution reference: https://www.algodale.com/problems/set-matrix-zeroes/
+'''
+class Solution:
+    '''
+    ### TC is O(m * n):
+    -  iterating through every cells, to find the zero cells. = O(m * n) 1️⃣
+    -  iterating through every cells, to update the rows. = O(m * n) 2️⃣
+    -  iterating through every cells, to update the columns. = O(m * n) 3️⃣
+
+    ### SC is O(m + n):
+    - using each set to store the rows(O(m)) and columns(O(n)) that have zero. = O(m + n)
+    '''
+    def setZeroesWithSet(self, matrix: List[List[int]]) -> None:
+        zero_rows = set() # SC: O(m)
+        zero_cols = set() # SC: O(n)
+
+        for r in range(len(matrix)): # TC: O(m * n)
+            for c in range(len(matrix[0])):
+                if matrix[r][c] == 0:
+                    zero_rows.add(r)
+                    zero_cols.add(c)
+
+        for r in zero_rows: # TC: O(m * n)
+            for i in range(len(matrix[0])):
+                matrix[r][i] = 0
+
+        for c in zero_cols: # TC: O(m * n)
+            for i in range(len(matrix)):
+                matrix[i][c] = 0
+
+    '''
+    ### TC is O(m * n):
+    - check if the first row or column has zero. = O(m + n)
+    - iterating through every cells, if it has zero, mark the first row and column. = O(m * n) 1️⃣
+    - update the matrix based on the marks(0) in the first row and column. = O(m * n) 2️⃣
+    - if the first row or column has zero, iterating through every cells, in the first row or column and updating it. = O(m + n) 
+
+    ### SC is O(1):
+    - using the first_row_has_zero and first_col_has_zero to store the zero information. = O(1)
+    '''
+    def setZeroesWithMarkerAndVariable(self, matrix: List[List[int]]) -> None:
+        rows = len(matrix)
+        cols = len(matrix[0])
+
+        first_row_has_zero = any(matrix[0][j] == 0 for j in range(cols)) # TC: O(n), SC: O(1)
+        first_col_has_zero = any(matrix[i][0] == 0 for i in range(rows)) # TC: O(m), SC: O(1)
+
+        for r in range(1, rows): # TC: O(m * n)
+            for c in range(1, cols):
+                if matrix[r][c] == 0:
+                    matrix[r][0] = 0
+                    matrix[0][c] = 0
+
+        for r in range(1, rows): # TC: O(m * n)
+            for c in range(1, cols):
+                if matrix[r][0] == 0 or matrix[0][c] == 0:
+                    matrix[r][c] = 0
+
+        if first_row_has_zero: 
+            for c in range(cols): # TC: O(n)
+                matrix[0][c] = 0
+
+        if first_col_has_zero:
+            for r in range(rows): # TC: O(m)
+                matrix[r][0] = 0
diff --git a/unique-paths/dusunax.py b/unique-paths/dusunax.py
new file mode 100644
index 000000000..dac03df5f
--- /dev/null
+++ b/unique-paths/dusunax.py
@@ -0,0 +1,21 @@
+'''
+# 62. Unique Paths
+
+use dynamic programming & a dp table to store the number of ways to reach each cell.
+
+### TC is O(m * n):
+- iterating through every cell in the grid. = O(m * n)
+- updating each cell in the grid. = O(1)
+
+### SC is O(m * n):
+- using a dp table (2D array) to store the number of ways to reach each cell. = O(m * n)
+'''
+class Solution:
+    def uniquePaths(self, m: int, n: int) -> int:
+        dp = [[1] * n for _ in range(m)]
+
+        for y in range(1, m):
+            for x in range(1, n):
+                dp[y][x] = dp[y - 1][x] + dp[y][x - 1]
+
+        return dp[m - 1][n - 1]