diff --git a/longest-substring-without-repeating-characters/hyejj19.ts b/longest-substring-without-repeating-characters/hyejj19.ts
new file mode 100644
index 0000000000..e9d88103ca
--- /dev/null
+++ b/longest-substring-without-repeating-characters/hyejj19.ts
@@ -0,0 +1,24 @@
+function lengthOfLongestSubstring(s: string): number {
+  if (s.length <= 1) return s.length;
+
+  let left = 0;
+  let right = 0;
+  let maxLen = -1;
+  const sMap = new Map();
+  while (right !== s.length) {
+    const curS = s[right];
+    // Map에 존재하지 않는다 -> 중복 X -> right 전진
+    if (!sMap.get(curS)) {
+      sMap.set(s[right], 1);
+      right++;
+    } else {
+      // Map에 존재 한다 -> 중복 O -> left 전진
+      while (left <= right && sMap.get(curS)) {
+        sMap.set(s[left], sMap.get(s[left]) - 1);
+        left++;
+      }
+    }
+    maxLen = Math.max(maxLen, right - left);
+  }
+  return maxLen;
+}
diff --git a/number-of-islands/hyejj19.ts b/number-of-islands/hyejj19.ts
new file mode 100644
index 0000000000..3409423e3f
--- /dev/null
+++ b/number-of-islands/hyejj19.ts
@@ -0,0 +1,42 @@
+// sc : O(n*m)
+// tc : O(n*m)
+function numIslands(grid: string[][]): number {
+  let count = 0;
+  const visited = Array.from({length: grid.length}, () =>
+    Array.from({length: grid[0].length}, () => false),
+  );
+  const directions = [
+    [-1, 0],
+    [1, 0],
+    [0, -1],
+    [0, 1],
+  ];
+
+  function explore(x, y) {
+    visited[x][y] = true;
+    for (let [dx, dy] of directions) {
+      const nx = dx + x;
+      const ny = dy + y;
+      if (
+        nx < grid.length &&
+        ny < grid[0].length &&
+        nx >= 0 &&
+        ny >= 0 &&
+        !visited[nx][ny] &&
+        grid[nx][ny] === '1'
+      ) {
+        explore(nx, ny);
+      }
+    }
+  }
+
+  for (let x = 0; x < grid.length; x++) {
+    for (let y = 0; y < grid[x].length; y++) {
+      if (!visited[x][y] && grid[x][y] === '1') {
+        count++;
+        explore(x, y);
+      }
+    }
+  }
+  return count;
+}
diff --git a/reverse-linked-list/hyejj19.ts b/reverse-linked-list/hyejj19.ts
new file mode 100644
index 0000000000..f95d78ffdb
--- /dev/null
+++ b/reverse-linked-list/hyejj19.ts
@@ -0,0 +1,18 @@
+/*
+time complexity : O(n)
+space complexity : O(1)
+*/
+function reverseList(head: ListNode | null): ListNode | null {
+  let prev = null;
+  let cur = head;
+  let next = null;
+
+  while (cur !== null) {
+    next = cur.next;
+    cur.next = prev;
+    prev = cur;
+    cur = next;
+  }
+
+  return prev;
+}