diff --git a/combination-sum/seungseung88.js b/combination-sum/seungseung88.js
new file mode 100644
index 000000000..7bb7a6f26
--- /dev/null
+++ b/combination-sum/seungseung88.js
@@ -0,0 +1,27 @@
+/**
+ * 시간복잡도: O(2^n)
+ * 공간복잡도: O(target)
+ */
+
+function combinationSum(canditates, target) {
+  const result = [];
+  const nums = [];
+
+  function dfs(start, total) {
+    if (total > target) return;
+    if (total === target) result.push([...nums]);
+
+    // 중복제거를 위해 start로 시작
+    for (let i = start; i < canditates.length; i += 1) {
+      num = canditates[i];
+      nums.push(num);
+      dfs(i, total + num);
+      nums.pop();
+    }
+  }
+
+  // 시작 인덱스, 누적 합
+  dfs(0, 0);
+
+  return result;
+}
diff --git a/decode-ways/seungseung88.js b/decode-ways/seungseung88.js
new file mode 100644
index 000000000..671bfec21
--- /dev/null
+++ b/decode-ways/seungseung88.js
@@ -0,0 +1,26 @@
+/**
+ * 시간 복잡도: O(n)
+ * 공간 복잡도: O(n)
+ */
+var numDecodings = function (s) {
+  const memo = new Map();
+  memo.set(s.length, 1);
+
+  function dfs(start) {
+    if (memo.has(start)) {
+      return memo.get(start);
+    }
+
+    if (s[start] === '0') {
+      memo.set(start, 0);
+    } else if (start + 1 < s.length && parseInt(s.slice(start, start + 2)) < 27) {
+      memo.set(start, dfs(start + 1) + dfs(start + 2));
+    } else {
+      memo.set(start, dfs(start + 1));
+    }
+
+    return memo.get(start);
+  }
+
+  return dfs(0);
+};
diff --git a/merge-two-sorted-lists/seungseung88.js b/merge-two-sorted-lists/seungseung88.js
new file mode 100644
index 000000000..9a0cc2616
--- /dev/null
+++ b/merge-two-sorted-lists/seungseung88.js
@@ -0,0 +1,23 @@
+/**
+ * 시간 복잡도: O(n + m)
+ * 공간 복잡도: O(1)
+ */
+var mergeTwoLists = function (list1, list2) {
+  const dummy = new ListNode();
+  node = dummy;
+
+  while (list1 && list2) {
+    if (list1.val < list2.val) {
+      node.next = list1;
+      list1 = list1.next;
+    } else {
+      node.next = list2;
+      list2 = list2.next;
+    }
+    node = node.next;
+  }
+
+  node.next = list1 || list2;
+
+  return dummy.next;
+};