diff --git a/contains-duplicate/kay30kim.cpp b/contains-duplicate/kay30kim.cpp
new file mode 100644
index 000000000..f00d7c7c4
--- /dev/null
+++ b/contains-duplicate/kay30kim.cpp
@@ -0,0 +1,77 @@
+/*
+Clarifications
+1) Is this sorted Array?
+2) What is the maximum / minimum value in nums?
+
+Solutions
+1) Brute force - two nested for loop 
+T(C): O(n^2)
+S(C): O(1)
+2) Sort - Mergesort or library sort(Merge+quick)
+T(C) : O(NlogN)
+S(C) : O(N)
+3) Hash
+T(C) : O(1)
+S(C) : O(N)
+*/
+
+class Solution {
+public:
+    // Solution 2-1) Library Sort
+    // bool containsDuplicate(vector<int>& nums) {
+    //     sort(nums.begin(), nums.end());
+    //     for (size_t i = 0; i < nums.size() - 1; i++)
+    //     {
+    //         if (nums[i] == nums[i + 1])
+    //             return true;
+    //     }
+    //     return false;
+    // }
+
+    // Solution 2-2) Implement Sort
+    void mergeSort(vector<int>& nums, int start, int end, vector<int>& temp)
+    {
+        if (start >= end)
+            return;
+        int mid = (start + end) / 2;
+        mergeSort(nums, start, mid, temp);
+        mergeSort(nums, mid + 1, end, temp);
+        int i = start, j = mid + 1, k = start;
+        while (i <= mid && j <= end)
+        {
+            if (nums[i] < nums[j])
+                temp[k++] = nums[i++];
+            else
+                temp[k++] = nums[j++];
+        }
+        while (i <= mid)
+            temp[k++] = nums[i++];
+        while (j <= end)
+            temp[k++] = nums[j++];
+        for (int p = start; p <= end; p++)
+            nums[p] = temp[p];
+    }
+    // Solution 1-2) Library Sort
+    // bool containsDuplicate(vector<int>& nums) {
+    //     vector<int> temp(nums.size(), 0);
+    //     mergeSort(nums, 0, nums.size() - 1, temp);
+    //     for (size_t i = 0; i < nums.size() - 1; i++)
+    //     {
+    //         if (nums[i] == nums[i + 1])
+    //             return true;
+    //     }
+    //     return false;
+    // }
+
+    // solution 2 - hash
+    bool containsDuplicate(vector<int>& nums) {
+        unordered_map<int, int> hash;
+        for (size_t i = 0; i < nums.size(); i++)
+        {
+            if (hash.find(nums[i]) != hash.end())
+                return true;
+            hash[nums[i]] = i;
+        }
+        return false;
+    }
+};
diff --git a/house-robber/kay30kim.cpp b/house-robber/kay30kim.cpp
new file mode 100644
index 000000000..219fc17c4
--- /dev/null
+++ b/house-robber/kay30kim.cpp
@@ -0,0 +1,14 @@
+class Solution {
+public:
+    int rob(vector<int>& nums) {
+        vector<vector<int>> dp(nums.size() + 1, vector<int>(2, 0));
+        dp[0][1] = nums[0];
+        for (int i = 1; i < nums.size(); i++)
+        {
+            dp[i][0] = max(dp[i - 1][0], dp[i - 1][1]);
+            dp[i][1] = dp[i - 1][0] + nums[i];
+        }
+        return max(dp[nums.size() - 1][0], dp[nums.size() - 1][1]);
+    }
+};
+//T(C) : O(N)
diff --git a/longest-consecutive-sequence/kay30kim.cpp b/longest-consecutive-sequence/kay30kim.cpp
new file mode 100644
index 000000000..b7ab16a28
--- /dev/null
+++ b/longest-consecutive-sequence/kay30kim.cpp
@@ -0,0 +1,22 @@
+class Solution {
+public:
+    int longestConsecutive(vector<int>& nums) {
+        unordered_set<int> isNum;
+        int ans = 0;
+        for (auto num : nums)
+            isNum.insert(num);
+        for (auto num : isNum) // (auto num : nums)하면 틀린다 일부러 시작하는 위치를 많이 넣는 테스트케이스 존재
+        {
+            if (isNum.find(num - 1) == isNum.end() && isNum.find(num) !=isNum.end())
+            {
+                int curLen = 0;
+                for (int seqNum = num; isNum.find(seqNum) != isNum.end(); seqNum++)
+                {
+                    curLen += 1;
+                }
+                ans = max(ans, curLen);
+            }
+        }
+        return ans;
+    }
+};
diff --git a/top-k-frequent-elements/kay30kim.cpp b/top-k-frequent-elements/kay30kim.cpp
new file mode 100644
index 000000000..0212e8f5f
--- /dev/null
+++ b/top-k-frequent-elements/kay30kim.cpp
@@ -0,0 +1,56 @@
+// 1) Sort -> val, cnt -> 
+// T(C) : O(NlogN)
+// 2) Heap
+// T(C) : O(NlogN)
+// 3) Bucket Sort
+// T(C) : O(N)
+class Solution {
+private:
+    struct Node
+    {
+        int val;
+        int cnt;
+        bool operator<(const Node &input) const 
+        {
+            return cnt < input.cnt;
+        }
+    };
+public:
+    // Solution 2 - Heap
+    // vector<int> topKFrequent(vector<int>& nums, int k) {
+    //     priority_queue<Node> pq;
+    //     unordered_map<int, int> numCnt;
+    //     vector<int> topKvalueVec;
+    //     for(auto num : nums)
+    //         numCnt[num] += 1;
+    //     for(auto it : numCnt)
+    //         pq.push({it.first, it.second});
+    //     for (int i = 0; i < k && !pq.empty(); i++)
+    //     {
+    //         topKvalueVec.push_back(pq.top().val);
+    //         pq.pop();
+    //     }
+    //     return topKvalueVec;
+    // }
+    
+    // Solution 3 - Bucket Sort
+    vector<int> topKFrequent(vector<int>& nums, int k) {
+        unordered_map<int,int> numToCnt;
+        vector<vector<int>> freq(nums.size() + 1, vector<int>());
+        vector<int> ans;
+        for (int i = 0; i < nums.size(); i++)
+            numToCnt[nums[i]] += 1;
+        for (auto it : numToCnt)
+            freq[it.second].push_back(it.first);
+        for (int cnt = nums.size(); cnt >= 0; cnt--)
+        {
+            for (auto num : freq[cnt])
+            {
+                ans.push_back(num);
+                if (ans.size() >= k)
+                    return ans;
+            }
+        }
+        return ans;
+    }
+};
diff --git a/two-sum/kay30kim.cpp b/two-sum/kay30kim.cpp
new file mode 100644
index 000000000..abbf5f042
--- /dev/null
+++ b/two-sum/kay30kim.cpp
@@ -0,0 +1,26 @@
+// 1) Brute Force - two nested for loop
+// T(C) : O(N^2)
+// 2) sort & two pointer
+// T(C) : O(NlogN)
+// 3) hash
+// T(C) : O(N)
+
+
+class Solution {
+public:
+    vector<int> twoSum(vector<int>& nums, int target) {
+        vector<int> indicesVector;
+        unordered_map<int, int> numToIdx;
+        for (int i = 0; i < nums.size(); i++)
+        {
+            if(numToIdx.find(target - nums[i]) != numToIdx.end())
+            {
+                indicesVector.push_back(numToIdx[target - nums[i]]);
+                indicesVector.push_back(i);
+                break;
+            }
+            numToIdx[nums[i]] = i;
+        }
+        return indicesVector;
+    }
+};