new file: P3379.cpp

P3379.cpp

@@ -0,0 +1,46 @@
+#include<bits/stdc++.h>
+using namespace std;
+int n, m, s;
+vector<int> fri[500005];
+int fa[500005][25];
+int dep[500005];
+void dfs(int now, int from = -1) {
+    for(int i = 1; i <= 24; ++i) {
+        fa[now][i] = fa[fa[now][i - 1]][i - 1];
+    }
+    for(int i : fri[now]) {
+        if(i == from) continue;
+        dep[i] = dep[now] + 1;
+        fa[i][0] = now;
+        dfs(i, now);
+    }
+}
+int lca(int a, int b) {
+    if(dep[a] < dep[b]) swap(a, b);
+    for(int i = 24; i >= 0 && dep[a] > dep[b]; --i) {
+        if(a == b) return a;
+        if(dep[fa[a][i]] >= dep[b] && dep[a] >= (1 << i)) a = fa[a][i];
+        if(a == b) return a;
+    }
+    if(a == b) return a;
+    for(int i = 24; i >= 0; --i) {
+        if(fa[a][i] != fa[b][i] && dep[a] >= (1 << i)) a = fa[a][i], b = fa[b][i];
+    }
+    if(a != b) a = fa[a][0];
+    return a;
+}
+int main() {
+    scanf("%d%d%d", &n, &m, &s);
+    for(int i = 1, u, v; i < n; ++i) {
+        scanf("%d%d", &u, &v);
+        fri[u].push_back(v);
+        fri[v].push_back(u);
+    }
+    dfs(s);
+    while(m--) {
+        int a, b;
+        scanf("%d%d", &a, &b);
+        printf("%d\n", lca(a, b));
+    }
+    return 0;
+}