diff --git a/product-of-array-except-self/changchanghwang.go b/product-of-array-except-self/changchanghwang.go
new file mode 100644
index 000000000..e6aaa51ac
--- /dev/null
+++ b/product-of-array-except-self/changchanghwang.go
@@ -0,0 +1,26 @@
+// time complexity: O(n)
+// space complexity: O(1)
+// prefix와 postfix를 이용하여 계산
+// 예를 들어, [1, 2, 3, 4] 일 때,
+// prefix는 [1, 1, 2, 6] 이고 postfix는 [24, 12, 4, 1] 이다.
+// 그리고 서로 곱하면 [24, 12, 8, 6] 이 된다.
+func productExceptSelf(nums []int) []int {
+	res := make([]int, len(nums))
+	for i := range res {
+		res[i] = 1
+	}
+
+	prefix := 1
+	for i := 0; i < len(nums); i++ {
+		res[i] = prefix
+		prefix *= nums[i]
+	}
+
+	postfix := 1
+	for i := len(nums) - 1; i >= 0; i-- {
+		res[i] *= postfix
+		postfix *= nums[i]
+	}
+
+	return res
+}
diff --git a/reverse-bits/changchanghwang.go b/reverse-bits/changchanghwang.go
new file mode 100644
index 000000000..9f604dae2
--- /dev/null
+++ b/reverse-bits/changchanghwang.go
@@ -0,0 +1,6 @@
+// Time Complexity: O(1)
+// Space Complexity: O(1)
+func reverseBits(num uint32) uint32 {
+	reversedBits := bits.Reverse32(num)
+	return reversedBits
+}
diff --git a/two-sum/changchanghwang.go b/two-sum/changchanghwang.go
new file mode 100644
index 000000000..7a4abf675
--- /dev/null
+++ b/two-sum/changchanghwang.go
@@ -0,0 +1,15 @@
+// Time: O(n)
+// Space: O(n)
+func twoSum(nums []int, target int) []int {
+	m := make(map[int]int)
+
+	// O(n)
+	for i, num := range nums {
+		// O(1)
+		if j, ok := m[target-num]; ok && j != i { // target = num2 + num1 -> num2 = target - num1 을 이용하여 두 수를 찾는다.
+			return []int{j, i}
+		}
+		m[num] = i // 없다면 현재 수를 키로 하여 인덱스를 저장한다.
+	}
+	return nil
+}