diff --git a/contains-duplicate/Hong-Study.cpp b/contains-duplicate/Hong-Study.cpp
new file mode 100644
index 0000000000..e9b624b678
--- /dev/null
+++ b/contains-duplicate/Hong-Study.cpp
@@ -0,0 +1,16 @@
+class Solution {
+public:
+    bool containsDuplicate(vector<int>& nums) {
+        std::unordered_map<int, int> map;
+        for(const auto& num: nums){
+            if(map.find(num) != map.end()){
+                return true;
+            }
+
+            map[num] = 1;
+        }
+
+        return false;
+    }
+};
+
diff --git a/longest-consecutive-sequence/Hong-Study.cpp b/longest-consecutive-sequence/Hong-Study.cpp
new file mode 100644
index 0000000000..a3fec66872
--- /dev/null
+++ b/longest-consecutive-sequence/Hong-Study.cpp
@@ -0,0 +1,20 @@
+class Solution {
+public:
+    int longestConsecutive(vector<int>& nums) {
+        std::set<int> sorts{nums.begin(), nums.end()};
+	
+	// 개선 필요...
+        int maxSequence = 0, sequence = 0, i = -1;
+        for (const auto& num : sorts) {
+            if (sorts.find(num + 1) != sorts.end()) {
+                sequence += 1;
+            } else {
+                maxSequence = std::max(maxSequence, sequence + 1);
+                sequence = 0;
+            }
+        }
+
+        return maxSequence;
+    }
+};
+
diff --git a/top-k-frequent-elements/Hong-Study.cpp b/top-k-frequent-elements/Hong-Study.cpp
new file mode 100644
index 0000000000..ebe4cc13df
--- /dev/null
+++ b/top-k-frequent-elements/Hong-Study.cpp
@@ -0,0 +1,29 @@
+class Solution {
+public:
+    vector<int> topKFrequent(vector<int>& nums, int k) {
+        std::unordered_map<int, int> maps;
+        for (const auto num : nums) {
+            if (maps.find(num) == maps.end())
+                maps[num] = 0;
+            maps[num] += 1;
+        }
+
+        std::vector<std::pair<int, int>> sorts{maps.begin(), maps.end()};
+        sort(sorts.begin(), sorts.end(),
+             [](std::pair<int, int> a, std::pair<int, int> b) {
+                 return a.second > b.second;
+             });
+
+        std::vector<int> result;
+        int i = 0;
+        for (const auto m : sorts) {
+            result.push_back(m.first);
+            i++;
+            if (i == k)
+                break;
+        }
+
+        return result;
+    }
+};
+
diff --git a/two-sum/Hong-Study.cpp b/two-sum/Hong-Study.cpp
new file mode 100644
index 0000000000..f03ca5612c
--- /dev/null
+++ b/two-sum/Hong-Study.cpp
@@ -0,0 +1,19 @@
+class Solution {
+public:
+    vector<int> twoSum(vector<int>& nums, int target) {
+        std::map<int, int> numMap;
+        for (int i = 0; i < nums.size(); i++) {
+            int leftNum = target - nums[i];
+            if (numMap.find(leftNum) == numMap.end()) {
+                numMap[nums[i]] = i;
+                continue;
+            }
+
+            int idx = numMap[leftNum];
+            return std::vector<int>{idx, i};
+        }
+
+        return std::vector<int>{0, 0};
+    }
+};
+