diff --git a/longest-substring-without-repeating-characters/daehyun99.py b/longest-substring-without-repeating-characters/daehyun99.py
new file mode 100644
index 0000000000..50fcaad914
--- /dev/null
+++ b/longest-substring-without-repeating-characters/daehyun99.py
@@ -0,0 +1,16 @@
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        l = 0
+        seen = set()
+        longest = 0
+
+        for c in s:
+            if c not in seen:
+                seen.add(c)
+                longest = max(longest, len(seen))
+            else:
+                while c in seen:
+                    seen.remove(s[l])
+                    l += 1
+                seen.add(c)
+        return longest
diff --git a/number-of-islands/daehyun99.py b/number-of-islands/daehyun99.py
new file mode 100644
index 0000000000..91d5ff40f7
--- /dev/null
+++ b/number-of-islands/daehyun99.py
@@ -0,0 +1,26 @@
+# Time: O(M * N)
+# Space: O(M * N)
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        h = len(grid)
+        w = len(grid[0])
+        count = 0
+        for i in range(h):
+            for j in range(w):
+                if grid[i][j] == "1":
+                    stack = []
+                    stack.append([i, j])
+                    while len(stack) > 0 :
+                        x, y = stack.pop()
+
+                        grid[x][y] = "0"
+                        if x > 0 and grid[x-1][y] == "1":
+                            stack.append([x-1, y])
+                        if x + 1 < h and grid[x+1][y] == "1":
+                            stack.append([x+1, y])
+                        if y > 0 and grid[x][y-1] == "1":
+                            stack.append([x, y-1])
+                        if y + 1 < w and grid[x][y+1] == "1":
+                            stack.append([x, y+1])
+                    count += 1
+        return count
diff --git a/reverse-linked-list/daehyun99.py b/reverse-linked-list/daehyun99.py
new file mode 100644
index 0000000000..ffe7b26315
--- /dev/null
+++ b/reverse-linked-list/daehyun99.py
@@ -0,0 +1,20 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+# Time: O(N)
+# Space: O(1)
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None:
+            return head
+        pointer = head
+        left = None
+        while pointer.next is not None:
+            right = pointer.next
+            pointer.next = left
+            left = pointer
+            pointer = right
+        pointer.next = left
+        return pointer
diff --git a/set-matrix-zeroes/daehyun99.py b/set-matrix-zeroes/daehyun99.py
new file mode 100644
index 0000000000..a2d0dbf5ec
--- /dev/null
+++ b/set-matrix-zeroes/daehyun99.py
@@ -0,0 +1,22 @@
+class Solution:
+    def setZeroes(self, matrix: List[List[int]]) -> None:
+        row = set()
+        col = set()
+
+        for i in range(len(matrix)):
+            if 0 in matrix[i]:
+                row.add(i)
+                for j in range(len(matrix[0])):
+                    if matrix[i][j] == 0:
+                        col.add(j)
+
+        for i in row:
+            matrix[i] = [0] * len(matrix[0])
+        
+        for i in range(len(matrix)):
+            if i in row:
+                continue
+            for j in range(len(matrix[0])):
+                if j in col:
+                    matrix[i][j] = 0
+
diff --git a/unique-paths/daehyun99.py b/unique-paths/daehyun99.py
new file mode 100644
index 0000000000..d577fd44f2
--- /dev/null
+++ b/unique-paths/daehyun99.py
@@ -0,0 +1,16 @@
+class Solution:
+    def uniquePaths(self, m: int, n: int) -> int:
+        matrix = [[0] * n for _ in range(m)]
+        matrix[0][0] += 1
+
+        for x in range(m):
+            for y in range(n):
+                if x == 0 and y == 0:
+                    continue
+                elif x == 0:
+                    matrix[x][y] += matrix[x][y-1]
+                elif y == 0:
+                    matrix[x][y] += matrix[x-1][y]
+                else:
+                    matrix[x][y] += (matrix[x][y-1] + matrix[x-1][y])
+        return matrix[m-1][n-1]