diff --git a/combination-sum/Real-Reason.kt b/combination-sum/Real-Reason.kt
new file mode 100644
index 000000000..bce40e84f
--- /dev/null
+++ b/combination-sum/Real-Reason.kt
@@ -0,0 +1,23 @@
+package leetcode_study
+
+fun combinationSum(candidates: IntArray, target: Int): List<List<Int>> {
+    val result = mutableListOf<List<Int>>()
+    val nums = ArrayDeque<Int>()
+    dfs(candidates, target, 0, 0, nums, result)
+
+    return result
+}
+
+private fun dfs(candidates: IntArray, target: Int, startIdx: Int, total: Int, nums: ArrayDeque<Int>, result: MutableList<List<Int>>) {
+    if (target < total) return
+    if (target == total) {
+        result.add(ArrayList(nums))
+        return
+    }
+    for (i in startIdx..< candidates.size) {
+        val num = candidates[i]
+        nums.add(num)
+        dfs(candidates, target, i, total + num, nums, result)
+        nums.removeLast()
+    }
+}
diff --git a/product-of-array-except-self/Real-Reason.kt b/product-of-array-except-self/Real-Reason.kt
new file mode 100644
index 000000000..039f93356
--- /dev/null
+++ b/product-of-array-except-self/Real-Reason.kt
@@ -0,0 +1,29 @@
+package leetcode_study
+
+fun productExceptSelf(nums: IntArray): IntArray {
+    // ex. nums = [1, 2, 3, 4]
+    val leftStartProducts = mutableListOf(1)
+    val rightStartProducts = mutableListOf(1)
+
+    // mutableNums = [1, 1, 2, 3, 4]
+    val mutableNums = nums.toMutableList()
+    mutableNums.add(0, 1)
+    mutableNums.add(1)
+
+    // leftStartProducts = [1, 1, 2, 6, 24, 24]
+    // rightStartProducts = [24, 24, 24, 12, 4, 1]
+    for (idx in 1..< mutableNums.size) {
+        val leftNum = mutableNums[idx]
+        val rightNum = mutableNums[mutableNums.size - 1 - idx]
+
+        leftStartProducts.add(leftStartProducts.last() * leftNum)
+        rightStartProducts.add(index = 0, element = rightStartProducts.first() * rightNum)
+    }
+
+    val result = mutableListOf<Int>()
+    for (idx in 0..mutableNums.size - 3) {
+        result.add(leftStartProducts[idx] * rightStartProducts[idx + 2])
+    }
+
+    return result.toIntArray()
+}
diff --git a/two-sum/Real-Reason.kt b/two-sum/Real-Reason.kt
new file mode 100644
index 000000000..84e1952f8
--- /dev/null
+++ b/two-sum/Real-Reason.kt
@@ -0,0 +1,16 @@
+package leetcode_study
+
+class `Real-Reason` {
+    fun twoSum(nums: IntArray, target: Int): IntArray {
+        for (startIdx in 0..< nums.size - 1) {
+            val firstNum = nums[startIdx]
+            for (endIdx in startIdx + 1..< nums.size) {
+                val secondNum = nums[endIdx]
+                if (target == firstNum + secondNum) {
+                    return intArrayOf(startIdx, endIdx)
+                }
+            }
+        }
+        throw RuntimeException("There is no solution")
+    }
+}