lca_rmq: repeated call of build() (#232)

* lca_rmq: repeated call of build()

* [auto-verifier] verify commit 157d934

Co-authored-by: GitHub <noreply@github.com>

Author: hitonanode

.verify-helper/timestamps.remote.json

@@ -220,7 +220,7 @@
 "tree/test/jump_on_tree.test.cpp": "2022-07-31 01:04:01 +0900",
 "tree/test/lca.test.cpp": "2022-07-31 01:04:01 +0900",
 "tree/test/lca.yuki898.test.cpp": "2022-07-31 01:04:01 +0900",
-"tree/test/lca_rmq.test.cpp": "2022-01-08 20:23:44 +0900",
+"tree/test/lca_rmq.test.cpp": "2022-09-24 13:23:24 +0900",
 "tree/test/rerooting.aoj1595.test.cpp": "2021-10-23 00:38:09 +0900",
 "tree/test/rerooting.yuki1718.test.cpp": "2021-10-23 00:38:09 +0900",
 "tree/test/tree_isomorphism.aoj1613.test.cpp": "2022-07-12 00:34:46 +0900",

tree/lca_rmq.hpp

@@ -5,12 +5,11 @@
 #include <utility>
 #include <vector>
 
-// CUT begin
 struct TreeLCA {
     const int N;
     std::vector<std::vector<int>> to;
-    bool built;
-    TreeLCA(int V = 0) : N(V), to(V), built(false) {}
+    int root;
+    TreeLCA(int V = 0) : N(V), to(V), root(-1) {}
 
     void add_edge(int u, int v) {
         assert(0 <= u and u < N);
@@ -37,25 +36,29 @@ struct TreeLCA {
     }
 
     StaticRMQ<P> rmq;
-    void build(int root = 0) {
-        assert(root >= 0 and root < N);
-        built = true;
-        subtree_begin.resize(N);
+    void build(int root_) {
+        assert(root_ >= 0 and root_ < N);
+        if (root == root_) return;
+        root = root_;
+        subtree_begin.assign(N, 0);
+        vis_order.clear();
         vis_order.reserve(N * 2);
-        depth.resize(N);
+        depth.assign(N, 0);
         _build_dfs(root, -1, 0);
         rmq = {vis_order, P{N, -1}};
     }
 
-    int lca(int u, int v) {
+    bool built() const noexcept { return root >= 0; }
+
+    int lca(int u, int v) const {
         assert(0 <= u and u < N);
         assert(0 <= v and v < N);
-        if (!built) build();
+        assert(built());
 
-        auto a = subtree_begin[u], b = subtree_begin[v];
+        int a = subtree_begin[u], b = subtree_begin[v];
         if (a > b) std::swap(a, b);
         return rmq.get(a, b + 1).second;
     };
 
-    int path_length(int u, int v) { return depth[u] + depth[v] - depth[lca(u, v)] * 2; }
+    int path_length(int u, int v) const { return depth[u] + depth[v] - depth[lca(u, v)] * 2; }
 };

tree/lca_rmq.md

@@ -7,6 +7,8 @@ documentation_of: ./lca_rmq.hpp
 
 ## 使用方法
 
+`add_edge(int u, int v)` を $N - 1$ 回行って木を構築した後，`build(int root)` によって根を指定する．
+
 ```cpp
 TreeLCA tree(N);
 for (int e = 0; e < N - 1; e++) {
@@ -15,6 +17,8 @@ for (int e = 0; e < N - 1; e++) {
     tree.add_edge(u, v);
 }
 
+tree.build(0);
+
 cout << tree.lca(a, b) << '\n'; // (a, b) の最長共通祖先
 cout << tree.path_length(a, b) << '\n'; // 2 頂点 a, b の距離
 ```

tree/test/lca_rmq.test.cpp

@@ -11,5 +11,7 @@ int main() {
     TreeLCA tree(N);
     for (int i = 1; i <= N - 1; i++) cin >> p, tree.add_edge(i, p);
 
+    tree.build(0);
+
     while (Q--) cout << tree.lca((cin >> u, u), (cin >> v, v)) << '\n';
 }