diff --git a/longest-substring-without-repeating-characters/JeonJe.java b/longest-substring-without-repeating-characters/JeonJe.java
new file mode 100644
index 0000000000..017ea650aa
--- /dev/null
+++ b/longest-substring-without-repeating-characters/JeonJe.java
@@ -0,0 +1,27 @@
+import java.util.*;
+
+// TC: O(n)
+// SC: O(min(n, m)) (m = 문자 집합 크기)
+class Solution {
+    public int lengthOfLongestSubstring(String s) {
+
+        int answer = 0;
+        int left = 0;
+
+        Set<Character> temp = new HashSet<>();
+
+        for (int right = 0; right < s.length(); right++) {
+
+            // 중복이 사라질 때까지 s[left]를 set에서 빼며 left를 전진시킨다.
+            while (temp.contains(s.charAt(right))) {
+                temp.remove(s.charAt(left));
+                left++;
+            }
+
+            temp.add(s.charAt(right));
+            answer = Math.max(answer, right - left + 1);
+        }
+
+        return answer;
+    }
+}
diff --git a/number-of-islands/JeonJe.java b/number-of-islands/JeonJe.java
new file mode 100644
index 0000000000..53d942dca0
--- /dev/null
+++ b/number-of-islands/JeonJe.java
@@ -0,0 +1,44 @@
+import java.util.*;
+
+// TC: O(m * n)
+// SC: O(m * n)
+class Solution {
+
+    private static int[] dx = {0, -1, 0, 1};
+    private static int[] dy = {1, 0, -1, 0};
+
+    private int n = 0;
+    private int m = 0;
+
+    public int numIslands(char[][] grid) {
+        n = grid[0].length;
+        m = grid.length;
+
+        boolean[][] visited = new boolean[m][n];
+
+        int answer = 0;
+
+        for (int i = 0; i < m; i++) {
+            for (int j = 0; j < n; j++) {
+                if (!visited[i][j] && grid[i][j] == '1') {
+                    answer++;
+                    dfs(i, j, grid, visited);
+                }
+            }
+        }
+        return answer;
+    }
+
+    private void dfs(int x, int y, char[][] grid, boolean[][] visited) {
+        visited[x][y] = true;
+
+        for (int i = 0; i < 4; i++) {
+            int nx = x + dx[i];
+            int ny = y + dy[i];
+
+            if (0 <= nx && nx < m && 0 <= ny && ny < n && grid[nx][ny] == '1' && !visited[nx][ny]) {
+                dfs(nx, ny, grid, visited);
+            }
+        }
+    }
+}
diff --git a/reverse-linked-list/JeonJe.java b/reverse-linked-list/JeonJe.java
new file mode 100644
index 0000000000..fcacc39f20
--- /dev/null
+++ b/reverse-linked-list/JeonJe.java
@@ -0,0 +1,19 @@
+import java.util.*;
+
+// TC: O(n)
+// SC: O(1)
+class Solution {
+    public ListNode reverseList(ListNode head) {
+        ListNode prev = null;
+        ListNode cur = head;
+
+        while (cur != null) {
+            ListNode temp = cur.next;
+            cur.next = prev;
+            prev = cur;
+            cur = temp;
+        }
+
+        return prev;
+    }
+}
diff --git a/unique-paths/JeonJe.java b/unique-paths/JeonJe.java
new file mode 100644
index 0000000000..f4f0cf3b38
--- /dev/null
+++ b/unique-paths/JeonJe.java
@@ -0,0 +1,19 @@
+import java.util.*;
+
+// TC: O(m * n)
+// SC: O(n)
+class Solution {
+    public int uniquePaths(int m, int n) {
+
+        int[] arr = new int[n];
+        Arrays.fill(arr, 1);
+
+        for (int i = 1; i < m; i++) {
+            for (int j = 1; j < n; j++) {
+                arr[j] = arr[j] + arr[j - 1];
+            }
+        }
+
+        return arr[n - 1];
+    }
+}