diff --git a/missing-number/reeseo3o.js b/missing-number/reeseo3o.js
new file mode 100644
index 0000000000..a1f8d6a3c3
--- /dev/null
+++ b/missing-number/reeseo3o.js
@@ -0,0 +1,9 @@
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+const missingNumber = (nums) => {
+  const n = nums.length;
+  const total = (n * (n + 1)) / 2; // 가우스 공식
+  const sum = nums.reduce((acc, cur) => acc + cur, 0);
+  return total - sum;
+};
diff --git a/reorder-list/reeseo3o.js b/reorder-list/reeseo3o.js
new file mode 100644
index 0000000000..7ea721fba3
--- /dev/null
+++ b/reorder-list/reeseo3o.js
@@ -0,0 +1,42 @@
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+const reorderList = (head) => {
+  if (!head || !head.next || !head.next.next) return;
+
+  // 1. 가운데 찾기 (slow는 앞쪽 리스트의 끝)
+  let slow = head;
+  let fast = head;
+  while (fast && fast.next) {
+    slow = slow.next;
+    fast = fast.next.next;
+  }
+
+  // 2. 뒷부분 뒤집기 (slow.next ~ 끝)
+  let prev = null;
+  let curr = slow.next;
+  slow.next = null; // 앞부분과 뒷부분 분리
+
+  while (curr) {
+    const next = curr.next;
+    curr.next = prev;
+    prev = curr;
+    curr = next;
+  }
+  // prev: 뒤집힌 두 번째 리스트의 head
+
+  // 3. 교차 머지 (head: 첫 번째, prev: 두 번째)
+  let first = head;
+  let second = prev;
+
+  while (second) {
+    const nextFirst = first.next;
+    const nextSecond = second.next;
+
+    first.next = second;
+    second.next = nextFirst;
+
+    first = nextFirst;
+    second = nextSecond;
+  }
+};