diff --git a/product-of-array-except-self/dalpang81.java b/product-of-array-except-self/dalpang81.java
new file mode 100644
index 000000000..9960f0570
--- /dev/null
+++ b/product-of-array-except-self/dalpang81.java
@@ -0,0 +1,23 @@
+/*
+* 시간복잡도: O(n)
+* 공간복잡도: O(1)
+* */
+class Solution {
+    public int[] productExceptSelf(int[] nums) {
+        int n = nums.length;
+        int[] answer = new int[n];
+
+        answer[0] = 1;
+        for (int i = 1; i < n; i++) {
+            answer[i] = answer[i - 1] * nums[i - 1];
+        }
+
+        int suffixProduct = 1;
+        for (int i = n - 1; i >= 0; i--) {
+            answer[i] *= suffixProduct;
+            suffixProduct *= nums[i];
+        }
+
+        return answer;
+    }
+}
diff --git a/reverse-bits/dalpang81.java b/reverse-bits/dalpang81.java
new file mode 100644
index 000000000..99050b039
--- /dev/null
+++ b/reverse-bits/dalpang81.java
@@ -0,0 +1,16 @@
+/*
+* 시간복잡도: O(1)
+* 공간복잡도: O(1)
+* */
+public class Solution {
+    // you need treat n as an unsigned value
+    public int reverseBits(int n) {
+        int result = 0;
+        for (int i = 0; i < 32; i++) {
+            result <<= 1;
+            result |= (n & 1);
+            n >>= 1;
+        }
+        return result;
+    }
+}
diff --git a/two-sum/dalpang81.java b/two-sum/dalpang81.java
new file mode 100644
index 000000000..1742ee8af
--- /dev/null
+++ b/two-sum/dalpang81.java
@@ -0,0 +1,23 @@
+/*
+* 시간복잡도 : O(n)
+* 공간복잡도 : O(n)
+* */
+
+import java.util.HashMap;
+import java.util.Map;
+
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        Map<Integer, Integer> map = new HashMap<>();
+
+        for (int i = 0; i < nums.length; i++) {
+            int complement = target - nums[i];
+
+            if (map.containsKey(complement)) {
+                return new int[] { map.get(complement), i };
+            }
+            map.put(nums[i], i);
+        }
+        return null;
+    }
+}