P3379 【模板】最近公共祖先（LCA）

R60590204

Luogu/problem/P3379/P3379.cpp

@@ -0,0 +1,60 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+
+int n, m, s, root, a, b, dep[500005], fa[500005][20];
+vector<int> tr[500005];
+
+void bfs(int root) {
+    memset(dep, 0x3f, sizeof(dep));
+    dep[0] = 0;
+    dep[root] = 1;
+    queue<int> q;
+    q.push(root);
+    while (!q.empty()) {
+        int t = q.front();
+        q.pop();
+        for (int i : tr[t]) {
+            if (dep[i] > dep[t] + 1) {
+                dep[i] = dep[t] + 1;
+                q.push(i);
+                fa[i][0] = t;
+                for (int k = 1; k <= 19; k++) {
+                    fa[i][k] = fa[fa[i][k - 1]][k - 1];
+                }
+            }
+        }
+    }
+}
+
+int lca(int a, int b) {
+    if (dep[a] < dep[b]) swap(a, b);
+    for (int k = 19; k >= 0; k--) {
+        if (dep[fa[a][k]] >= dep[b]) {
+            a = fa[a][k];
+        }
+    }
+    if (a == b) return a;
+    for (int k = 19; k >= 0; k--) {
+        if (fa[a][k] != fa[b][k]) {
+            a = fa[a][k];
+            b = fa[b][k];
+        }
+    }
+    return fa[a][0];
+}
+
+int main() {
+    cin >> n >> m >> root;
+    for (int i = 1; i < n; i++) {
+        cin >> a >> b;
+        tr[a].push_back(b);
+        tr[b].push_back(a);
+    }
+    bfs(root);
+    for (int i = 0; i < m; i++) {
+        cin >> a >> b;
+        cout << lca(a, b) << endl;
+    }
+    return 0;
+}