diff --git a/BaekJoon/11438/gwangseok.py b/BaekJoon/11438/gwangseok.py
new file mode 100644
index 0000000..8a568f2
--- /dev/null
+++ b/BaekJoon/11438/gwangseok.py
@@ -0,0 +1,77 @@
+import sys
+input = sys.stdin.readline
+from collections import defaultdict, deque
+
+
+class Node:
+    def __init__(self, num, depth, num_parents):
+        self.num = num
+        self.depth = depth
+        self.parents = [-1] * num_parents
+
+
+def find_lca(nodes, num_parents, node1, node2):
+    if nodes[node1].depth < nodes[node2].depth:
+        # node1의 depth를 더 깊게 설정
+        node1, node2 = node2, node1
+
+    depth_diff = nodes[node1].depth - nodes[node2].depth
+    if depth_diff != 0:
+        bit_diff = bin(depth_diff)[:1:-1]  # bit만 역순으로 만듦.
+        for idx, cur_bit in enumerate(bit_diff):
+            if cur_bit == '1': 
+                node1 = nodes[node1].parents[idx]
+    
+    if node1 == node2:
+        return node1
+
+    for idx in range(num_parents - 1, -1, -1):
+        if nodes[node1].parents[idx] != nodes[node2].parents[idx]:
+            node1 = nodes[node1].parents[idx]
+            node2 = nodes[node2].parents[idx]
+    
+    return nodes[node1].parents[0]
+
+
+N = int(input())
+graph = defaultdict(list)
+
+for _ in range(N-1):
+    node1, node2 = map(int, input().split())
+    graph[node1].append(node2)
+    graph[node2].append(node1)
+
+num_parents = 0
+v = 1
+while v < N:
+    v *= 2
+    num_parents += 1
+
+nodes = [None] * (N + 1)  # [i] = [depth, node_parent]
+nodes[1] = Node(1, 0, num_parents)
+
+is_visited = [False] * (N + 1)
+q = deque([])
+q.append(1)
+is_visited[1] = True
+
+
+while q:
+    cur_node = q.popleft()
+    for next_node in graph[cur_node]:
+        if is_visited[next_node] is False:
+            is_visited[next_node] = True
+            q.append(next_node)
+            nodes[next_node] = Node(next_node, nodes[cur_node].depth + 1, num_parents)
+            nodes[next_node].parents[0] = cur_node
+
+for parent_idx in range(1, num_parents):
+    for node_idx in range(2, N+1):
+        half_jump_node = nodes[node_idx].parents[parent_idx - 1]
+        nodes[node_idx].parents[parent_idx] = nodes[half_jump_node].parents[parent_idx - 1]           
+
+
+M = int(input())
+for _ in range(M):
+    node1, node2 = map(int, input().split())
+    print(find_lca(nodes, num_parents, node1, node2))