new file: P3398.cpp

P3398.cpp

@@ -0,0 +1,73 @@
+#include<bits/stdc++.h>
+using namespace std;
+int n, q;
+vector<int> fri[100005];
+int fa[100005];
+int dep[100005];
+int top[100005];
+int son[100005];
+int cnt[100005];
+void dfs1(int now, int from) {
+    cnt[now] = 1;
+    for(int i : fri[now]) {
+        if(i == from) continue;
+        fa[i] = now;
+        dep[i] = dep[now] + 1;
+        dfs1(i, now);
+        cnt[now] += cnt[i];
+        if(cnt[son[now]] < cnt[i]) son[now] = i;
+    }
+}
+void dfs2(int now, int from) {
+    top[now] = from;
+    if(son[now]) dfs2(son[now], from);
+    for(int i : fri[now]) {
+        if(i == fa[now] || i == son[now]) continue;
+        dfs2(i, i);
+    }
+}
+int lca(int a, int b) {
+    while(top[a] != top[b]) {
+        if(dep[top[a]] < dep[top[b]]) swap(a, b);
+        a = fa[top[a]];
+    }
+    if(dep[a] < dep[b]) swap(a, b);
+    return b;
+}
+int main() {
+    scanf("%d%d", &n, &q);
+    for(int i = 1, u, v; i < n; ++i) {
+        scanf("%d%d", &u, &v);
+        fri[u].push_back(v);
+        fri[v].push_back(u);
+    }
+    dfs1(1, 0);
+    dfs2(1, 1);
+    while(q--) {
+        int a, b, c, d;
+        scanf("%d%d%d%d", &a, &b, &c, &d);
+        int lca1 = lca(a, b);
+        int lca2 = lca(c, d);
+        // lca1 在 [c, d] 上或 lca2 在 [a, b] 上
+        if(dep[lca2] >= dep[lca1]) {
+            if(lca(a, lca2) == lca2 || lca(b, lca2) == lca2) {
+                printf("Y\n");
+            } else {
+                printf("N\n");
+            }
+        } else {
+            if(lca(c, lca1) == lca1 || lca(d, lca1) == lca1) {
+                printf("Y\n");
+            } else {
+                printf("N\n");
+            }
+        }
+    }
+    return 0;
+}
+/*
+    - deep[x]>=deep[LCA(s,t)]
+
+    - LCA(s,x)=x或LCA(t,x)=x;
+
+*/