diff --git a/longest-substring-without-repeating-characters/yuseok89.py b/longest-substring-without-repeating-characters/yuseok89.py
new file mode 100644
index 0000000000..2c0aaafdd5
--- /dev/null
+++ b/longest-substring-without-repeating-characters/yuseok89.py
@@ -0,0 +1,19 @@
+# TC: O(N)
+# SC: O(K)
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        idx_map = {}
+        start = 0
+        ans = 0
+
+        for end in range(len(s)):
+            c = s[end]
+            if c in idx_map and start <= idx_map[c]:
+                start = idx_map[c] + 1
+            else:
+                ans = max(ans, end - start + 1)
+
+            idx_map[c] = end
+
+        return ans
+
diff --git a/number-of-islands/yuseok89.py b/number-of-islands/yuseok89.py
new file mode 100644
index 0000000000..d30efbc2e4
--- /dev/null
+++ b/number-of-islands/yuseok89.py
@@ -0,0 +1,28 @@
+# TC: O(NM)
+# SC: O(NM)
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        ans = 0
+        n = len(grid)
+        m = len(grid[0])
+
+        dirs = [[-1, 0], [1, 0], [0, -1], [0, 1]]
+
+        def fill(row: int, col: int):
+            grid[row][col] = '0'
+
+            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 grid[new_row][new_col] == '1':
+                    fill(new_row, new_col)
+
+        for i in range(0, n):
+            for j in range(0, m):
+                if grid[i][j] == '1':
+                    fill(i, j)
+                    ans += 1
+
+        return ans
+
diff --git a/reverse-linked-list/yuseok89.py b/reverse-linked-list/yuseok89.py
new file mode 100644
index 0000000000..36d7d96f8b
--- /dev/null
+++ b/reverse-linked-list/yuseok89.py
@@ -0,0 +1,21 @@
+# TC: O(N)
+# SC: O(1)
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+
+        prev = None
+        cur = head
+
+        while cur:
+            nxt = cur.next
+            cur.next = prev
+            prev = cur
+            cur = nxt
+
+        return prev
+
diff --git a/set-matrix-zeroes/yuseok89.py b/set-matrix-zeroes/yuseok89.py
new file mode 100644
index 0000000000..08949bf4b6
--- /dev/null
+++ b/set-matrix-zeroes/yuseok89.py
@@ -0,0 +1,25 @@
+# TC: O(NM)
+# SC: O(N+M)
+class Solution:
+    def setZeroes(self, matrix: List[List[int]]) -> None:
+        """
+        Do not return anything, modify matrix in-place instead.
+        """
+
+        n = len(matrix)
+        m = len(matrix[0])
+
+        row_set = set()
+        col_set = set()
+
+        for row in range(0, n):
+            for col in range(0, m):
+                if matrix[row][col] == 0:
+                    row_set.add(row)
+                    col_set.add(col)
+
+        for row in range(0, n):
+            for col in range(0, m):
+                if row in row_set or col in col_set:
+                    matrix[row][col] = 0
+
diff --git a/unique-paths/yuseok89.py b/unique-paths/yuseok89.py
new file mode 100644
index 0000000000..74e162d177
--- /dev/null
+++ b/unique-paths/yuseok89.py
@@ -0,0 +1,14 @@
+# TC: O(NM)
+# SC: O(N)
+class Solution:
+    def uniquePaths(self, m: int, n: int) -> int:
+
+        cnt = [0] * n
+        cnt[0] = 1
+
+        for i in range(0, m):
+            for j in range(1, n):
+                cnt[j] += cnt[j - 1]
+
+        return cnt[n - 1]
+