diff --git a/non-overlapping-intervals/hwi-middle.cpp b/non-overlapping-intervals/hwi-middle.cpp
new file mode 100644
index 0000000000..ee517d5a57
--- /dev/null
+++ b/non-overlapping-intervals/hwi-middle.cpp
@@ -0,0 +1,28 @@
+class Solution {
+public:
+    int eraseOverlapIntervals(vector<vector<int>>& intervals) {
+        sort(intervals.begin(), intervals.end(), [](vector<int>& a, vector<int>& b) {
+            return a[1] < b[1];
+        });
+
+        int ans = 0;
+        int k = INT_MIN;
+        
+        for (int i = 0; i < intervals.size(); i++) 
+        {
+            int s = intervals[i][0];
+            int e = intervals[i][1];
+            
+            if (s >= k) 
+            {
+                k = e;
+            } 
+            else 
+            {
+                ans++;
+            }
+        }
+        
+        return ans;
+    }
+};
diff --git a/number-of-connected-components-in-an-undirected-graph/hwi-middle.cpp b/number-of-connected-components-in-an-undirected-graph/hwi-middle.cpp
new file mode 100644
index 0000000000..c05a854675
--- /dev/null
+++ b/number-of-connected-components-in-an-undirected-graph/hwi-middle.cpp
@@ -0,0 +1,51 @@
+class Solution {
+public:
+    int countComponents(int n, vector<vector<int>>& edges) {
+        int components = n;
+        vector<int> p(n, -1);
+        for (auto& e : edges)
+        {
+            if (uni(p, e[0], e[1])) // 이 엣지를 통해 합쳐지면
+            {
+               components--; // 컴포넌트 수는 하나 감소
+            }
+        }
+
+        return components;
+    }
+
+    // union-find
+    int find(vector<int>&p, int x)
+    {
+        if(p[x] < 0)
+        {
+            return x;
+        }
+
+        return p[x] = find(p, p[x]);
+    }
+
+    bool uni(vector<int>&p, int u, int v)
+    {
+        u = find(p, u);
+        v = find(p, v);
+
+        if (u == v)
+        {
+            return false;
+        }
+
+        if (p[v] < p[u])
+        {
+            swap(u, v);
+        }
+
+        if (p[u] == p[v])
+        {
+            p[u]--;
+        }
+
+        p[v] = u;
+        return true;
+    }
+};
diff --git a/remove-nth-node-from-end-of-list/hwi-middle.cpp b/remove-nth-node-from-end-of-list/hwi-middle.cpp
new file mode 100644
index 0000000000..609ca9b05b
--- /dev/null
+++ b/remove-nth-node-from-end-of-list/hwi-middle.cpp
@@ -0,0 +1,40 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* removeNthFromEnd(ListNode* head, int n) {
+        // 스택에 넣고 뒤집어서 n번째 노드 찾기
+        stack<ListNode*> s;
+        ListNode* cur = head;
+        while (cur != nullptr)
+        {
+            s.push(cur);
+            cur = cur->next;
+        }
+
+        for (int i = 0; i < n; ++i)
+        {
+            cur = s.top();
+            s.pop();
+        }
+
+        // 스택이 빌 때 까지 pop한 경우는 head를 제거하는 경우
+        if (s.empty())
+        {
+            return head->next;
+        }
+
+        // 그 외에는 prev가 존재
+        ListNode* prev = s.top();
+        prev->next = cur->next;
+        return head;
+    }
+};
diff --git a/same-tree/hwi-middle.cpp b/same-tree/hwi-middle.cpp
new file mode 100644
index 0000000000..bb63fbb074
--- /dev/null
+++ b/same-tree/hwi-middle.cpp
@@ -0,0 +1,28 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+
+// 재귀를 통해 동일한 트리인지 확인
+class Solution {
+public:
+    bool isSameTree(TreeNode* p, TreeNode* q) {
+        if (p == nullptr || q == nullptr)
+        {
+            return p == q;
+        }
+
+        bool l = isSameTree(p->left, q->left);
+        bool r = isSameTree(p->right, q->right);
+        bool cur = p->val == q->val;
+
+        return l && r && cur;
+    }
+};
diff --git a/serialize-and-deserialize-binary-tree/hwi-middle.cpp b/serialize-and-deserialize-binary-tree/hwi-middle.cpp
new file mode 100644
index 0000000000..fa8ba544bd
--- /dev/null
+++ b/serialize-and-deserialize-binary-tree/hwi-middle.cpp
@@ -0,0 +1,116 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+ * };
+ */
+
+ // 그래야할 필요는 없다고 명시하지만, LeetCode 형식과 동일하게 직렬화/역직렬화 구현
+class Codec {
+public:
+    // Encodes a tree to a single string.
+    string serialize(TreeNode* root) {
+        vector<string> tokens;
+        queue<TreeNode*> q;
+        q.push(root);
+        while (!q.empty()) 
+        {
+            auto node = q.front();
+            q.pop();
+            if (node == nullptr) 
+            {
+                tokens.push_back("null");
+                continue;
+            }
+
+            tokens.push_back(to_string(node->val));
+            q.push(node->left);
+            q.push(node->right);
+        }
+
+        // 굳이 지우지 않아도 역직렬화가 가능
+        // 하지만 LeetCode와 동일하게 구현하기 위한 처리
+        while (!tokens.empty() && tokens.back() == "null")
+        {
+            tokens.pop_back();
+        }
+
+        string s = "[";
+        for (int i = 0; i < tokens.size(); i++) 
+        {
+            if (i)
+            { 
+                s += ",";
+            
+            }
+            s += tokens[i];
+        }
+
+        s += "]";
+        //cout << s;
+        return s;
+    }
+
+    // Decodes your encoded data to tree.
+    TreeNode* deserialize(string data) {
+        if (data == "[]")
+        {
+            return nullptr;
+        }
+
+        data = data.substr(1, data.size() - 2); // '[', ']' 제거
+        stringstream ss(data);
+        string token;
+
+        queue<string> v;
+        while (getline(ss, token, ','))
+        {
+            v.push(token);
+        }
+
+        TreeNode* root = new TreeNode(stoi(v.front()));
+        v.pop();
+        queue<TreeNode*> q;
+        q.push(root);
+        while (!q.empty())
+        {
+            auto cur = q.front();
+            q.pop();
+
+            // 만약 직렬화 과정에서 null을 제거하지 않았다면 이러한 확인은 필요없어짐
+            if (v.empty())
+            {
+                break;
+            }
+
+            string l = v.front();
+            v.pop();
+            if (l != "null")
+            {
+                cur->left = new TreeNode(stoi(l));
+                q.push(cur->left);
+            }
+
+            if (v.empty())
+            {
+                break;
+            }
+            
+            string r = v.front();
+            v.pop();
+            if (r != "null")
+            {
+                cur->right = new TreeNode(stoi(r));
+                q.push(cur->right);
+            }
+        }
+        return root;
+    }
+};
+
+// Your Codec object will be instantiated and called as such:
+// Codec ser, deser;
+// TreeNode* ans = deser.deserialize(ser.serialize(root));