diff --git a/contains-duplicate/mkwkw.java b/contains-duplicate/mkwkw.java
new file mode 100644
index 000000000..c12d84a41
--- /dev/null
+++ b/contains-duplicate/mkwkw.java
@@ -0,0 +1,23 @@
+//1. using map
+//2. using set : why? It doesn't have to use a pair of key and value.
+
+class Solution {
+    public boolean containsDuplicate(int[] nums) {
+        //Map<Integer, Boolean> appearance = new HashMap<>();
+        Set<Integer> appearance = new HashSet<>();
+
+        for(int i=0; i<nums.length; i++)
+        {
+            if(appearance.contains(nums[i]))
+            {
+                return true;
+            }
+            else
+            {
+                appearance.add(nums[i]);
+            }
+        }
+
+        return false;
+    }
+}
diff --git a/two-sum/mkwkw.cpp b/two-sum/mkwkw.cpp
new file mode 100644
index 000000000..7d90286ac
--- /dev/null
+++ b/two-sum/mkwkw.cpp
@@ -0,0 +1,43 @@
+//Store the number and the index.
+//There can be only 2 same numbers. <- there is exactly one solution
+
+class Solution {
+public:
+    vector<int> twoSum(vector<int>& nums, int target) {
+        
+        map<int, vector<int>> numAndIndex;
+        vector<int> answer;
+
+        //map: key:nums[i], value:{indexs}
+        for(int i=0; i<nums.size(); i++)
+        {
+            numAndIndex[nums[i]].push_back(i);
+        }
+
+        for(int i=0; i<nums.size()-1; i++)
+        {
+            if(numAndIndex.contains(target-nums[i]))
+            {
+                //To pick another number, not own number
+                if(target-nums[i]==nums[i]&&numAndIndex[nums[i]].size()==2)
+                {
+                    answer.push_back(numAndIndex[nums[i]][0]);
+                    answer.push_back(numAndIndex[nums[i]][1]);
+                }
+                else if(target-nums[i]!=nums[i])
+                {
+                    answer.push_back(numAndIndex[nums[i]][0]);
+                    answer.push_back(numAndIndex[target-nums[i]][0]);
+                }
+            }
+
+            if(answer.size()==2)
+            {
+                break;
+            }
+        }
+        
+        return answer;
+
+    }
+};