diff --git a/missing-number/YoungSeok-Choi.java b/missing-number/YoungSeok-Choi.java
new file mode 100644
index 000000000..21ed3d39f
--- /dev/null
+++ b/missing-number/YoungSeok-Choi.java
@@ -0,0 +1,20 @@
+
+class Solution {
+    public int missingNumber(int[] nums) {
+
+        int len = nums.length;
+        Map<Integer, Boolean> nMap = new HashMap<>();
+
+        for (int i = 0; i <= len; i++) {
+            nMap.put(i, true);
+        }
+
+        for (int anInt : nums) {
+            nMap.remove(anInt);
+        }
+
+        List<Integer> keyList = new ArrayList<>(nMap.keySet());
+
+        return keyList.get(0);
+    }
+}
diff --git a/number-of-connected-components-in-an-undirected-graph/YoungSeok-Choi.java b/number-of-connected-components-in-an-undirected-graph/YoungSeok-Choi.java
new file mode 100644
index 000000000..eed208df7
--- /dev/null
+++ b/number-of-connected-components-in-an-undirected-graph/YoungSeok-Choi.java
@@ -0,0 +1,84 @@
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+// 문제 해결에 용이하게 주어진 단방향 그래프를 양방향으로 바꾸어 DFS 실행
+class Solution {
+
+    public int countComponents(int n, int[][] edges) {
+        List<List<Integer>> graph = new ArrayList<>();
+        boolean[] visited = new boolean[n];
+
+        // 인접 리스트 초기화
+        for (int i = 0; i < n; i++) {
+            graph.add(new ArrayList<>());
+        }
+
+        // 양방향 그래프 구성
+        for (int[] edge : edges) {
+            graph.get(edge[0]).add(edge[1]);
+            graph.get(edge[1]).add(edge[0]);
+        }
+
+        int count = 0;
+
+        for (int i = 0; i < n; i++) {
+            if (!visited[i]) {
+                dfs(i, graph, visited);
+                count++;
+            }
+        }
+
+        return count;
+    }
+
+    private void dfs(int node, List<List<Integer>> graph, boolean[] visited) {
+        visited[node] = true;
+        for (int neighbor : graph.get(node)) {
+            if (!visited[neighbor]) {
+                dfs(neighbor, graph, visited);
+            }
+        }
+    }
+}
+
+// NOTE: 문제에서 주어진 무방향 그래프를 탐색하다, 앞 순번 DFS에서 연결 요소로 판단한 정점을 뒤 DFS 에서 다시 방문한 경우의
+// 처리가 까다로웠다
+class WrongSolution {
+
+    Map<Integer, Boolean> visitMap = new HashMap<>();
+    boolean[] visit;
+    public int cnt = 0;
+
+    public int countComponents(int n, int[][] edges) {
+
+        visit = new boolean[n];
+
+        for (int i = 0; i < n; i++) {
+            if (!visit[i]) {
+                visitMap = new HashMap<>();
+                cnt++;
+                dfs(i, edges);
+
+            }
+        }
+
+        return cnt;
+    }
+
+    public void dfs(int v, int[][] edges) {
+        visit[v] = true;
+        visitMap.put(v, true);
+
+        for (int i = 0; i < edges.length; i++) {
+            if (edges[i][0] == v && !visit[edges[i][1]]) {
+                dfs(edges[i][1], edges);
+            } else if (edges[i][0] == v && visit[edges[i][1]]) {
+                if (!visitMap.containsKey(edges[i][1])) {
+                    cnt--;
+                }
+            }
+        }
+    }
+}
diff --git a/remove-nth-node-from-end-of-list/YoungSeok-Choi.java b/remove-nth-node-from-end-of-list/YoungSeok-Choi.java
new file mode 100644
index 000000000..0f2d67d17
--- /dev/null
+++ b/remove-nth-node-from-end-of-list/YoungSeok-Choi.java
@@ -0,0 +1,48 @@
+public class ListNode {
+    int val;
+    ListNode next;
+
+    ListNode() {
+    }
+
+    ListNode(int val) {
+        this.val = val;
+    }
+
+    ListNode(int val, ListNode next) {
+        this.val = val;
+        this.next = next;
+    }
+}
+
+class Solution {
+    public ListNode removeNthFromEnd(ListNode head, int n) {
+        int length = 0;
+        ListNode cur = head;
+
+        while (cur != null) {
+            cur = cur.next;
+            length++;
+        }
+
+        int start = 1;
+        int targetIdx = length - n + 1;
+        ListNode result = new ListNode(-1);
+        ListNode c = result;
+
+        while (head != null) {
+            if (targetIdx == start) {
+                head = head.next;
+                start++;
+                continue;
+            }
+
+            c.next = new ListNode(head.val);
+            c = c.next;
+            start++;
+            head = head.next;
+        }
+
+        return result.next;
+    }
+}
diff --git a/reorder-list/YoungSeok-Choi.java b/reorder-list/YoungSeok-Choi.java
new file mode 100644
index 000000000..c505d5652
--- /dev/null
+++ b/reorder-list/YoungSeok-Choi.java
@@ -0,0 +1,101 @@
+public class ListNode {
+    int val;
+    ListNode next;
+
+    ListNode() {
+    }
+
+    ListNode(int val) {
+        this.val = val;
+    }
+
+    ListNode(int val, ListNode next) {
+        this.val = val;
+        this.next = next;
+    }
+}
+
+class Solution {
+    public void reorderList(ListNode head) {
+        List<Integer> nArr = new ArrayList<>();
+        Map<Integer, List<ListNode>> lMap = new HashMap<>();
+
+        ListNode cur = head.next;
+        while (cur != null) {
+            ListNode next = cur.next;
+            cur.next = null;
+            nArr.add(cur.val);
+
+            if (lMap.containsKey(cur.val)) {
+                lMap.get(cur.val).add(cur);
+            } else {
+                List<ListNode> temp = new ArrayList<>();
+                temp.add(cur);
+                lMap.put(cur.val, temp);
+            }
+
+            cur = next;
+        }
+
+        int[] arr = nArr.stream().mapToInt(i -> i).toArray();
+        int start = 0;
+        int end = arr.length - 1;
+
+        while (start < end) {
+            head.next = lMap.get(arr[end]).remove(0);
+            head = head.next;
+            head.next = lMap.get(arr[start]).remove(0);
+            head = head.next;
+
+            start++;
+            end--;
+        }
+
+        if (arr.length % 2 != 0) {
+            for (int anInt : new ArrayList<>(lMap.keySet())) {
+                List<ListNode> anArr = lMap.get(anInt);
+
+                if (anArr.size() > 0) {
+                    head.next = anArr.remove(0);
+                }
+            }
+        }
+    }
+}
+
+// NOTE: 동일한 val의 ListNode를 고려하지 못하여 틀린 경우
+class WrongSolution {
+    public void reorderList(ListNode head) {
+        List<Integer> nArr = new ArrayList<>();
+        Map<Integer, ListNode> lMap = new HashMap<>();
+
+        ListNode cur = head.next;
+        while (cur != null) {
+            ListNode next = cur.next;
+            cur.next = null;
+            nArr.add(cur.val);
+            lMap.put(cur.val, cur);
+            cur = next;
+        }
+
+        int[] arr = nArr.stream().mapToInt(i -> i).toArray();
+        int start = 0;
+        int end = arr.length - 1;
+
+        while (start < end) {
+            head.next = lMap.remove(arr[end]);
+            head = head.next;
+            head.next = lMap.remove(arr[start]);
+            head = head.next;
+
+            start++;
+            end--;
+        }
+
+        if (arr.length % 2 != 0) {
+            for (int anInt : new ArrayList<>(lMap.keySet())) {
+                head.next = lMap.remove(anInt);
+            }
+        }
+    }
+}
diff --git a/same-tree/YoungSeok-Choi.java b/same-tree/YoungSeok-Choi.java
new file mode 100644
index 000000000..d110aaf4d
--- /dev/null
+++ b/same-tree/YoungSeok-Choi.java
@@ -0,0 +1,34 @@
+class TreeNode {
+    int val;
+    TreeNode left;
+    TreeNode right;
+
+    TreeNode() {
+    }
+
+    TreeNode(int val) {
+        this.val = val;
+    }
+
+    TreeNode(int val, TreeNode left, TreeNode right) {
+        this.val = val;
+        this.left = left;
+        this.right = right;
+    }
+}
+
+class Solution {
+    public boolean isSameTree(TreeNode p, TreeNode q) {
+
+        if (p == null && q == null) {
+            return true;
+        }
+
+        if (p == null || q == null || q.val != p.val) {
+            return false;
+        }
+
+        // 특정 깊이의 Node 가 같다면 null을 만날때 까지 탐색하고, 같지 않다면 바로 false를 반환.
+        return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
+    }
+}