diff --git a/S2OJ/109/109.cpp b/S2OJ/109/109.cpp
new file mode 100644
index 00000000..8adf0148
--- /dev/null
+++ b/S2OJ/109/109.cpp
@@ -0,0 +1,56 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+
+int n, m, s, x, y;
+
+struct node {
+    int depth, father;
+    vector<int> next;
+} tree[500005];
+bool vis[500005];
+
+void dfs(int depth, int root, int father) {
+    tree[root].father = father;
+    for (int i = 0; i < tree[root].next.size(); i++) {
+        if (tree[root].next[i] != father) {
+            tree[tree[root].next[i]].depth = depth + 1;
+            tree[tree[root].next[i]].father = root;
+            dfs(depth + 1, tree[root].next[i], root);
+        }
+    }
+    return;
+}
+
+void dfs2(int now) {
+    vis[now] = true;
+    if(now == s) return;
+    else dfs2(tree[now].father);
+}
+
+int dfs3(int now) {
+    if(vis[now]) return now;
+    else return dfs3(tree[now].father);
+}
+
+int main() {
+    scanf("%d%d%d", &n, &m, &s);
+    for (int i = 1; i < n; i++) {
+        scanf("%d%d", &x, &y);
+        tree[x].next.push_back(y);
+        tree[y].next.push_back(x);
+    }
+    dfs(1, s, s);
+    for(int i = 0 ; i < m ; i++) {
+        scanf("%d%d", &x, &y);
+        memset(vis, 0x00, sizeof(vis));
+        if(tree[x].depth > tree[y].depth) {
+            dfs2(x);
+            printf("%d\n", dfs3(y));
+        } else {
+            dfs2(y);
+            printf("%d\n", dfs3(x));
+        }
+    }
+    return 0;
+}