diff --git a/Jongeun/Day15/1046_LastStoneWeight.cpp b/Jongeun/Day15/1046_LastStoneWeight.cpp
new file mode 100644
index 0000000..aafd33a
--- /dev/null
+++ b/Jongeun/Day15/1046_LastStoneWeight.cpp
@@ -0,0 +1,49 @@
+#include <iostream>
+#include <vector>
+#include <queue>
+
+using namespace std;
+
+class Solution
+{
+public:
+    int lastStoneWeight(vector<int> &stones)
+    {
+        priority_queue<int> mpq;
+        for (auto i : stones)
+        {
+            mpq.push(i);
+        }
+
+        while (mpq.size() > 1)
+        {
+            int m1 = mpq.top();
+            mpq.pop();
+            int m2 = mpq.top();
+            mpq.pop();
+
+            if (abs(m1 - m2) != 0)
+            {
+                int m3 = abs(m1 - m2);
+                mpq.push(m2);
+            }
+        }
+
+        if (mpq.empty())
+        {
+            return 0;
+        }
+        else
+        {
+            return mpq.top();
+        }
+    }
+};
+
+int main()
+{
+    Solution sol;
+    vector<int> stones = {2, 7, 4, 1, 8, 1};
+    cout << sol.lastStoneWeight(stones) << endl;
+    return 0;
+}
diff --git a/Jongeun/Day15/208_ImplementTriePrefixTree.cpp b/Jongeun/Day15/208_ImplementTriePrefixTree.cpp
new file mode 100644
index 0000000..29e138c
--- /dev/null
+++ b/Jongeun/Day15/208_ImplementTriePrefixTree.cpp
@@ -0,0 +1,116 @@
+#include <iostream>
+#include <vector>
+#include <string>
+
+using namespace std;
+
+/*
+    Implement trie (store/retrieve keys in dataset of strings)
+
+    Each node contains pointer to next letter & is word flag
+
+    Time: O(n) insert, O(n) search, O(n) startsWith
+    Space: O(n) insert, O(1) search, O(1) startsWith
+*/
+
+class TrieNode
+{
+public:
+    TrieNode *children[26];
+    bool isWord;
+
+    TrieNode()
+    {
+        for (int i = 0; i < 26; i++)
+        {
+            children[i] = NULL;
+        }
+        isWord = false;
+    }
+};
+
+class Trie
+{
+public:
+    Trie()
+    {
+        root = new TrieNode();
+    }
+
+    void insert(string word)
+    {
+        TrieNode *node = root;
+        int curr = 0;
+
+        for (int i = 0; i < word.size(); i++)
+        {
+            curr = word[i] - 'a';
+            if (node->children[curr] == NULL)
+            {
+                node->children[curr] = new TrieNode();
+            }
+            node = node->children[curr];
+        }
+
+        node->isWord = true;
+    }
+
+    bool search(string word)
+    {
+        TrieNode *node = root;
+        int curr = 0;
+
+        for (int i = 0; i < word.size(); i++)
+        {
+            curr = word[i] - 'a';
+            if (node->children[curr] == NULL)
+            {
+                return false;
+            }
+            node = node->children[curr];
+        }
+
+        return node->isWord;
+    }
+
+    bool startsWith(string prefix)
+    {
+        TrieNode *node = root;
+        int curr = 0;
+
+        for (int i = 0; i < prefix.size(); i++)
+        {
+            curr = prefix[i] - 'a';
+            if (node->children[curr] == NULL)
+            {
+                return false;
+            }
+            node = node->children[curr];
+        }
+
+        return true;
+    }
+
+private:
+    TrieNode *root;
+};
+
+int main()
+{
+    Trie *obj = new Trie();
+    obj->insert("apple");
+    cout << obj->search("apple") << endl;   // 1
+    cout << obj->search("app") << endl;     // 0
+    cout << obj->startsWith("app") << endl; // 1
+    obj->insert("app");
+    cout << obj->search("app") << endl; // 1
+    return 0;
+}
+
+/**
+ * Your Trie object will be instantiated and called as such:
+ * Trie* obj = new Trie();
+ * obj->insert(word);
+ * bool param_2 = obj->search(word);
+ * bool param_3 = obj->startsWith(prefix);
+ */
diff --git a/Jongeun/Day15/39_CombinationSum.cpp b/Jongeun/Day15/39_CombinationSum.cpp
new file mode 100644
index 0000000..f3fb3b4
--- /dev/null
+++ b/Jongeun/Day15/39_CombinationSum.cpp
@@ -0,0 +1,71 @@
+#include <iostream>
+#include <vector>
+#include <unordered_map>
+using namespace std;
+
+class Solution
+{
+public:
+    vector<vector<int>> combinationSum(vector<int> &candidates, int target)
+    {
+
+        vector<vector<int>> result;
+        // sort(candidates.begin(),candidates.end());
+
+        unordered_map<int, int> comb;
+        for (auto i : candidates)
+        {
+            comb[i] = 0;
+        }
+        _combinationSum(result, candidates, comb, target, 0);
+        return result;
+    }
+
+    void _combinationSum(vector<vector<int>> &result, vector<int> &candidates, unordered_map<int, int> &comb, int left, int step)
+    {
+        if (left == 0)
+        {
+            vector<int> temp;
+            for (auto num : comb)
+            {
+                for (int i = 0; i < num.second; i++)
+                {
+                    temp.push_back(num.first);
+                }
+            }
+            result.push_back(temp);
+            return;
+        }
+        else if (left < 0 || step >= candidates.size())
+        {
+            return;
+        }
+
+        int freq = 0;
+
+        while (left >= 0)
+        {
+            comb[candidates[step]] = freq;
+            left -= candidates[step] * freq;
+            _combinationSum(result, candidates, comb, left, step + 1);
+            freq++;
+        }
+    }
+};
+
+int main()
+{
+    Solution obj;
+    vector<int> candidates = {2, 3, 6, 7};
+    int target = 7;
+    vector<vector<int>> result = obj.combinationSum(candidates, target);
+    for (auto i : result)
+    {
+        for (auto j : i)
+        {
+            cout << j << " ";
+        }
+        cout << endl;
+    }
+    return 0;
+}