WA on #11 & #20

提交记录：Link。

做法是边分治 + 虚树，$O(n \log n)$。

求助大佬！回复必回关！

#include <bits/stdc++.h>
#define int long long 
using namespace std;

const int N = 3e6 + 10;
const int L = 23;
const int I = 4e18 + 10;
int n, _n;
int h1[N], h2[N], h3[N], h4[N];
int t1 = 1, t2 = 1, t3 = 1, t4 = 1;
int d1[N], d2[N], d[N];
int dfn[N], tim;
int sz[N], cl[N];
int nd[N], ct;
bool vis[N];
int ans = -I;
int stk[N], top;
int f[N][2];
vector <int> v1, v2, p, ep;

struct edge{
    int to, nxt, val;
}e1[N], e2[N], e3[N], e4[N];
struct ST{
    int tp, f[N][L], lg[N];

    int gt(int x, int y) { return dfn[x] < dfn[y] ? x : y; }
    void init() {
        lg[0] = -1;
        for (int i = 1; i < N; i++) lg[i] = lg[i >> 1] + 1;
        for (int j = 1; j < L; j++) {
            for (int i = 1; i + (1 << j) - 1 <= n; i++) {
                f[i][j] = gt(f[i][j - 1], f[i + (1 << (j - 1))][j - 1]);
            }
        }
    }
    int lca(int u, int v) {
        if (u == v) return u;
        u = dfn[u], v = dfn[v];
        if (u > v) swap(u, v);
        int k = lg[v - u++];
        return gt(f[u][k], f[v - (1 << k) + 1][k]);
    }
}st;

void add1(int u, int v, int w) {
    e1[++t1] = {v, h1[u], w};
    h1[u] = t1;
}
void add2(int u, int v, int w) {
    e2[++t2] = {v, h2[u], w};
    h2[u] = t2;
}
void add3(int u, int v, int w) {
    e3[++t3] = {v, h3[u], w};
    h3[u] = t3;
}
void add4(int u, int v) {
    e4[++t4] = {v, h4[u]};
    h4[u] = t4;
}
void dfs1(int u, int fa) {
    for (int i = h1[u]; i; i = e1[i].nxt) {
        int v = e1[i].to, w = e1[i].val;
        if (v == fa) continue;
        d1[v] = d1[u] + w;
        dfs1(v, u);
    }
}
void dfs2(int u, int fa) {
    st.f[dfn[u] = ++tim][0] = fa;
    for (int i = h2[u]; i; i = e2[i].nxt) {
        int v = e2[i].to, w = e2[i].val;
        if (v == fa) continue;
        d2[v] = d2[u] + w; 
        dfs2(v, u);
    }
}
void rbd(int u, int fa) {
    int tp = 0;
    for (int i = h1[u]; i; i = e1[i].nxt) {
        int v = e1[i].to, w = e1[i].val;
        if (v == fa) continue;
        if (!tp) {
            add3(u, v, w), add3(v, u, w);
            tp = u;
        } else {
            int k = ++_n;
            add3(tp, k, 0), add3(k, tp, 0);
            add3(k, v, w), add3(v, k, w);
            tp = k;
        }
        rbd(v, u);
    }
}
void dfs3(int u, int fa) {
    sz[u] = 1;
    for (int i = h3[u]; i; i = e3[i].nxt) {
        int v = e3[i].to;
        if (vis[i] || v == fa) continue;
        dfs3(v, u);
        sz[u] += sz[v];
    }
}
void dfs4(int u, int fa, int rt, int &mn, int &g) {
    for (int i = h3[u]; i; i = e3[i].nxt) {
        int v = e3[i].to;
        if (vis[i] || v == fa) continue;
        dfs4(v, u, rt, mn, g);
        int mx = max(sz[v], sz[rt] - sz[v]);
        if (mx < mn) mn = mx, g = i;
    }
}
void dfs5(int u, int fa, int tp) {
    if (u <= n) cl[u] = tp, nd[++ct] = u;
    for (int i = h3[u]; i; i = e3[i].nxt) {
        int v = e3[i].to, w = e3[i].val;
        if (vis[i] || v == fa) continue;
        d[v] = d[u] + w;
        dfs5(v, u, tp);
    }
}
bool cmp(int x, int y) { return dfn[x] < dfn[y]; }
void bld() {
    p.resize(v1.size() + v2.size());
    merge(v1.begin(), v1.end(), v2.begin(), v2.end(), p.begin(), cmp);
    t4 = top = 0, stk[++top] = 1;
    for (int i = 0; i < p.size(); i++) {
        int u = p[i], dfu = dfn[p[i]];
        if (u == 1) continue;
        int v = stk[top], lc = st.lca(u, v);
        if (lc == v) {
            stk[++top] = u;
            continue;
        }
        while (stk[top] != lc) {
            v = stk[top--];
            if (dfn[stk[top]] < dfn[lc]) {
                stk[++top] = lc;
            }
            add4(stk[top], v), add4(v, stk[top]);
        }
        stk[++top] = u;
    }
    while (top > 1) {
        int v = stk[top--];
        add4(stk[top], v), add4(v, stk[top]);
    }
}
void dfs6(int u, int fa, int &res) {
    // cout << u << ' ';
    f[u][0] = f[u][1] = -I;
    if (cl[u] == 1) f[u][0] = d[u] + d1[u];
    if (cl[u] == 2) f[u][1] = d[u] + d1[u];
    for (int i = h4[u]; i; i = e4[i].nxt) {
        int v = e4[i].to;
        if (v == fa) continue;
        dfs6(v, u, res);
        res = max({res, f[u][0] + f[v][1] - 2 * d2[u], f[u][1] + f[v][0] - 2 * d2[u]});
        f[u][0] = max(f[u][0], f[v][0]);
        f[u][1] = max(f[u][1], f[v][1]);
    }
    h4[u] = 0;
}
vector <int> solve(int nu, int ts) {
    // cout << nu << ' ' << ts << '\n';
    vector <int> tv1, tv2;
    if (ts == 1) {
        if (nu > n) return ep;
        p.clear(), p.push_back(nu);
        return p;
    }
    dfs3(nu, 0);
    int mn = 1e9, g = 0;
    dfs4(nu, 0, nu, mn, g);
    if (!g) return ep;
    int u = e3[g].to, v = e3[g ^ 1].to;
    vis[g] = vis[g ^ 1] = 1;    
    if (sz[v] > sz[u]) tv1 = solve(u, sz[u]), tv2 = solve(v, ts - sz[u]);
    else tv1 = solve(u, n - sz[v]), tv2 = solve(v, sz[v]);
    d[u] = d[v] = ct = 0;
    dfs5(u, 0, 1), dfs5(v, 0, 2);
    int res = -I;
    v1 = tv1, v2 = tv2;
    bld(), dfs6(1, 0, res);
    ans = max(ans, res + e3[g].val);
    for (int i = 1; i <= ct; i++) cl[nd[i]] = 0;
    vis[g] = vis[g ^ 1] = 0;
    return p;
}

signed main() {
    ios :: sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    cin >> n;
    for (int i = 1; i < n; i++) {
        int u, v, w;
        cin >> u >> v >> w;
        add1(u, v, w), add1(v, u, w);
    }
    for (int i = 1; i < n; i++) {
        int u, v, w;
        cin >> u >> v >> w;
        add2(u, v, w), add2(v, u, w);
    }
    _n = n, rbd(1, 0);
    dfs1(1, 0), dfs2(1, 0);
    st.init();
    solve(1, _n);
    ans /= 2;
    for (int i = 1; i <= n; i++) {
        ans = max(ans, d1[i] - d2[i]);
    }
    cout << ans << '\n';
    return 0;
}