diff --git a/maximum-depth-of-binary-tree/YoungSeok-Choi.java b/maximum-depth-of-binary-tree/YoungSeok-Choi.java
new file mode 100644
index 000000000..77b892b48
--- /dev/null
+++ b/maximum-depth-of-binary-tree/YoungSeok-Choi.java
@@ -0,0 +1,35 @@
+import java.util.LinkedList;
+import java.util.Queue;
+
+// 시간복잡도 O(n)
+class Solution {
+    public int depth = 0;
+    public int maxDepth(TreeNode root) {
+
+        if(root == null) {
+            return depth;
+        }
+        
+        Queue<TreeNode> q = new LinkedList<>();
+        q.add(root);
+
+        while(!q.isEmpty()) {
+            int size = q.size();
+            depth++;
+
+            for(int i = 0; i < size; i++) {
+                TreeNode p = q.poll();
+
+                if(p.right != null) {
+                    q.add(p.right);
+                }
+
+                if(p.left != null) {
+                    q.add(p.left);
+                }
+            }
+        }
+
+        return depth;
+    }
+}
diff --git a/merge-two-sorted-lists/YoungSeok-Choi.java b/merge-two-sorted-lists/YoungSeok-Choi.java
new file mode 100644
index 000000000..10164c9ef
--- /dev/null
+++ b/merge-two-sorted-lists/YoungSeok-Choi.java
@@ -0,0 +1,44 @@
+// 시간복잡도 O(N + M)
+class Solution {
+
+    public ListNode root = null;
+    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
+
+        while(list1 != null || list2 != null) {
+            int v1 = 9999;
+            int v2 = 9999;
+            
+            if(list1 != null) {
+                v1 = list1.val;
+            }
+
+            if(list2 != null) {
+                v2 = list2.val;
+            }
+
+            if(v1 < v2) {
+                addNode(v1);
+                list1 = list1.next;    
+            } else {
+                addNode(v2);
+                list2 = list2.next;
+            }      
+        }
+
+        return root;
+    }
+
+    public void addNode (int val) {
+        if(root == null) {
+            root = new ListNode(val);
+            return;
+        }
+
+        ListNode now = root;
+        while(now.next != null) {
+            now = now.next;
+        }
+
+        now.next = new ListNode(val);
+    }
+}