I'm writing this blog just to record down some personal notes for solved problems:

ABC 133F : segment tree to solve tree problem

First of all, we can observe that queries can be done offline and the only thing that matters for each query is the color.

Lets do each color one by one, we can observe that distance between two nodes in the query = original distance & mdash; sum of edge weight of color $$$x$$$ + number of edges that have color $$$x$$$ in the path.

Now let us try to find what can be done.We can see that the number of edges with color $$$x$$$ going out of subtree $$$u$$$ will $$$ + 1$$$ if the starting point is from subtree $$$u$$$.So we can just $$$ + 1$$$ to all nodes inside subtree $$$u$$$.Then we can see that number of edges from node $$$x$$$ to $$$y$$$.Similarly, we can do $$$ + W$$$ for weight sum.

distance in original graph, edge number and weight sum in the path can be found in $$$O(N log N)$$$ using binary lifting and a dfs function.

This reduces the problem to $$$O(N ^ 2)$$$, lets optimize it!

We can try finding a better way to do the $$$ + 1$$$ and $$$ + W$$$ operations.Well, if you have learnt dfn, you should be able to continue from here.If we represent $$$tin_{ i }$$$ be the time we entered subtree $$$i$$$ and $$$tout_{ i }$$$ we left subtree $$$i$$$.Then we can use this to observe that we need to do $$$ + val$$$ in a range and find a value at a point $$$p$$$. * Yes, segment tree!*

However, we cannot just declare a new segment tree each time.We need to reuse the segment tree.So, we'll have to do $$$-1$$$ and $$$-W$$$ operations to revert the segment tree, or do something like range set.

Code: ~~~~~ // Problem: F — Colorful Tree // Memory Limit: 1024 MB // Time Limit: 4000 ms

//闲敲棋子落灯花//

include<bits/stdc++.h>

using namespace std; using i64 = long long;

struct edge { int u, v, color, w; }; struct query { int u, v, color, new_w, id; }; const int N = 1e5 + 50, K = 30; vector<pair<int, int>> G[N]; int dep[N], sp[N][K], val[N], ti, tin[N], tout[N]; void dfs(int u, int p) { tin[u] = ++ti; dep[u] = dep[p] + 1; sp[u][0] = p; for (int i = 1; i < K; i++)sp[u][i] = sp[sp[u][i — 1]][i — 1]; for (auto v : G[u]) if (v.first != p) { val[v.first] = val[u] + v.second; dfs(v.first, u); } tout[u] = ti; } int lca(int u, int v) { if (dep[u] < dep[v]) swap(u, v); for (int i = K — 1; i >= 0; i--)if (dep[sp[u][i]] >= dep[v])u = sp[u][i]; if (u == v) return u; for (int i = K — 1; i >= 0; i--)if (sp[u][i] != sp[v][i])u = sp[u][i], v = sp[v][i]; return sp[v][0]; } int dis(int u, int v) { int l = lca(u, v); return val[u] + val[v] — 2 * val[l]; } struct segtree { struct node { i64 v; friend node operator + (node a, node b) { node t; t.v = a.v + b.v; return t; } }; vector tr; vector tag; int SZ = 0; void apply_node(int p, int x) { tr[p].v += x; tag[p] += x; } void pushdown(int p) { if (tag[p] != 0) { apply_node(p << 1, tag[p]); apply_node(p << 1 | 1, tag[p]); tag[p] = 0; } return; } segtree(int N) { SZ = N + 200; tr.resize(SZ * 4); tag.assign(SZ * 4, 0); } #define ls p<<1 #define rs p<<1|1 void update(int x, int v) { update(1, 1, SZ — 100, v, x, x); } void update(int x, int y, int v) { update(1, 1, SZ — 100, v, x, y); } void update(int p, int l, int r, int v, int lq, int rq) { //cout << "updating: " << p << ' ' << l << ' ' << r << ' ' << lq << ' ' << rq <<" with value: "<<v<< '\n'; if (lq <= l && r <= rq) { apply_node(p, v); return; } int mid = (l + r) >> 1; pushdown(p); if (lq <= mid) update(ls, l, mid, v, lq, rq); if (rq >= mid + 1) update(rs, mid + 1, r, v, lq, rq); tr[p] = tr[ls] + tr[rs]; } node query(int x) { return query(1, 1, SZ — 100, x, x); } node query(int x, int y) { return query(1, 1, SZ — 100, x, y); } node query(int p, int l, int r, int lq, int rq) { //cout << "querying: " << p << ' ' << l << ' ' << r << ' ' << lq << ' ' << rq << '\n'; if (lq <= l && r <= rq) { return tr[p]; } int mid = (l + r) >> 1; pushdown(p); if (lq <= mid && rq >= mid + 1) return query(ls, l, mid, lq, rq) + query(rs, mid + 1, r, lq, rq); if (lq <= mid) return query(ls, l, mid, lq, rq); if (rq >= mid + 1) return query(rs, mid + 1, r, lq, rq); } }; //update/query -> point or range int main() { ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);

int N, Q; cin >> N >> Q;
vector<vector<edge>> C(N + 1);
for (int i = 0; i < N - 1; i++) {
    int a, b, c, d; cin >> a >> b >> c >> d;
    G[a].push_back({ b,d });
    G[b].push_back({ a,d }); //original graph
    C[c].push_back({ a,b,c,d }); //push into the edge set with color c
}
dfs(1, 0);
vector<vector<query>> Qp(N + 1);
vector<int> ans(Q + 1);
for (int i = 0; i < Q; i++) {
    int a, b, c, d; cin >> a >> b >> c >> d;
    ans[i] = dis(c, d);
    Qp[a].push_back({ c,d,a,b,i });
}
segtree segcnt(N), segval(N); // two segment trees, one countring the number of edges and the other counting the sum of weights
for (int i = 1; i <= N; i++) {
    for (auto p : C[i]) { // for each edge that has the color i
        int u = p.u, v = p.v, w = p.w;
        if (dep[u] > dep[v]) swap(u, v);//make sure we swap to do the +1 operation in the son not the parent
        segcnt.update(tin[v], tout[v], 1);
        segval.update(tin[v], tout[v], w);
    }

    for (auto p : Qp[i]) { // for each query that has color i
        int u = p.u, v = p.v, w = p.new_w;
        if (dep[u] > dep[v]) swap(u, v);
        int l = lca(u, v);
        int cnt = segcnt.query(tin[u]).v + segcnt.query(tin[v]).v - 2 * segcnt.query(tin[l]).v;
        int val = segval.query(tin[u]).v + segval.query(tin[v]).v - 2 * segval.query(tin[l]).v;
        ans[p.id] -= val;
        ans[p.id] += cnt * p.new_w;
    }

    for (auto p : C[i]) {
        int u = p.u, v = p.v, w = p.w;
        if (dep[u] > dep[v]) swap(u, v);
        segcnt.update(tin[v], tout[v], -1);
        segval.update(tin[v], tout[v], -w);
    }
}
for (int i = 0; i < Q; i++) {
    cout << ans[i] << '\n';
}

} ~~~~~