预处理出$dis_i$表示点$i$到根$1$的距离，答案是$dis_x+dis_y-2dis_{lca(x,y)}$

非常容易证明

代码如下

#include <bits/stdc++.h>
using namespace std;
int n, k, b[1000005], x, y, z, tot, h[500005], len[500005], fa[500005][33], dep[500005], lg[500005],
    f[1000005], ans;
struct node {
    int to, next, w;
} a[1000005];
void dfs(long long x, long long fn, long long l) {
    fa[x][0] = fn;
    dep[x] = dep[fn] + 1;
    len[x] = l;
    for (int i = 1; i <= 31; i++) {
        fa[x][i] = fa[fa[x][i - 1]][i - 1];
    }
    for (int i = h[x]; i; i = a[i].next) {
        if (a[i].to != fn) {
            dfs(a[i].to, x, l + a[i].w);
        }
    }
}
int lca(int x, int y) {
    if (dep[x] < dep[y]) {
        swap(x, y);
    }
    while (dep[x] > dep[y]) {
        x = fa[x][lg[dep[x] - dep[y]] - 1];
    }
    if (x == y) {
        return x;
    }
    for (int k = lg[dep[x]] - 1; k >= 0; k--) {
        if (fa[x][k] != fa[y][k]) {
            x = fa[x][k], y = fa[y][k];
        }
    }
    return fa[x][0];
}
void add(int x, int y, int z) {
    ++tot;
    a[tot].to = y;
    a[tot].next = h[x];
    a[tot].w = z;
    h[x] = tot;
}
void answer(int x, int fn) {
    for (int i = h[x]; i; i = a[i].next) {
        if (a[i].to != fn) {
            answer(a[i].to, x);
            f[x] += f[a[i].to];
        }
    }
    ans = max(ans, f[x]);
}
int main() {
    cin >> n;
    for (int i = 1; i <= n; ++i) {
        lg[i] = lg[i - 1] + (1 << lg[i - 1] == i);
    }
    for (int i = 1; i <= n - 1; i++) {
        cin >> x >> y >> z;
        add(x, y, z);
        add(y, x, z);
    }
    dfs(1, 0, 0);
    cin >> k;
    for (int i = 1; i <= k; i++) {
        cin >> x >> y;
        int t = lca(x, y);
        cout << len[x] + len[y] - 2 * len[t] << endl;
    }
}