diff --git a/insert-interval/Seoya0512.py b/insert-interval/Seoya0512.py
new file mode 100644
index 0000000000..113447e329
--- /dev/null
+++ b/insert-interval/Seoya0512.py
@@ -0,0 +1,26 @@
+'''
+Time Complexity : O(N)
+- newInterval을 삽입할 위치 탐색 O(N)
+- Intervals 병합을 위해 for문으로 모든 구간을 순회 O(N)
+- O(N) + O(N) = O(N)
+
+Space Complexity : O(N)
+- output 리스트에 모든 구간을 저장 O(N)
+
+'''
+class Solution:
+    def insert(self, intervals: List[List[int]], newInterval: List[int]) -> List[List[int]]:
+        idx = 0
+        while idx < len(intervals) and intervals[idx][0] < newInterval[0]:
+            idx +=1
+        intervals.insert(idx, newInterval)    # newInterval을 삽입할 위치 탐색 (O(N))
+
+        # Intervals 병합
+        output = [intervals[0]]
+        for interval in intervals[1:]:
+            # 이전 구간과 겹치는 경우
+            if output[-1][1] >= interval[0]:
+                output[-1][1] = max(output[-1][1], interval[1])
+            else:
+                output.append(interval)
+        return output
diff --git a/kth-smallest-element-in-a-bst/Seoya0512.py b/kth-smallest-element-in-a-bst/Seoya0512.py
new file mode 100644
index 0000000000..c90af8b43d
--- /dev/null
+++ b/kth-smallest-element-in-a-bst/Seoya0512.py
@@ -0,0 +1,29 @@
+'''
+Time Complexity: O(N)
+- dfs 함수가 모든 노드를 중위순회 방식으로 방문함므로 O(N) 소요
+
+Space Complexity: O(N)
+- aligned_arr 리스트에 모든 노드의 값을 저장하므로 O(N) 소요
+- 재귀 호출 스택이 최대 트리의 높이만큼 쌓일 수 있으므로 최악의 경우 O(N) 소요
+'''
+
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
+        aligned_arr = []
+
+        def dfs(node):
+            if not node:
+                return
+            dfs(node.left)
+            aligned_arr.append(node.val)
+            dfs(node.right)
+        
+        dfs(root)
+
+        return aligned_arr[k - 1]
diff --git a/lowest-common-ancestor-of-a-binary-search-tree/Seoya0512.py b/lowest-common-ancestor-of-a-binary-search-tree/Seoya0512.py
new file mode 100644
index 0000000000..de5bb29c1f
--- /dev/null
+++ b/lowest-common-ancestor-of-a-binary-search-tree/Seoya0512.py
@@ -0,0 +1,12 @@
+'''
+Time Complexity: O(H)
+- H는 트리의 높이
+'''
+
+class Solution:
+    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
+        if p.val < root.val and q.val < root.val:
+            return self.lowestCommonAncestor(root.left, p, q)
+        if p.val > root.val and q.val > root.val:
+            return self.lowestCommonAncestor(root.right, p, q)
+        return root
diff --git a/meeting-rooms/Seoya0512.py b/meeting-rooms/Seoya0512.py
new file mode 100644
index 0000000000..e3b469ff24
--- /dev/null
+++ b/meeting-rooms/Seoya0512.py
@@ -0,0 +1,19 @@
+'''
+Time Complexity: O(N log N)
+- intervals.sort() 는 O(N log N) 소요
+- for loop 는 O(N) 소요
+- 전체적으로 O(N log N) + O(N) = O(N log N)
+
+Space Complexity: O(1)
+- end_time, start_time 상수 변수 추가로 사용
+'''
+def can_attend_meetings(self, intervals: List[Interval]) -> bool:
+  intervals.sort()
+  end_time = intervals[0][1]
+
+  for i in range(1, len(intervals)):
+    start_time = intervals[i][0]
+    if end_time > start_time:
+      return False
+    end_time = intervals[i][1]
+  return True