#include <iostream>
#include <vector>
#include <queue>
#include <cstring>

using namespace std;

#define int long long
const int N = 300010;

struct edge {
    int y, w;
};

int n, s, t;
pair<int, int> p;
vector<edge> v[N];
int fa[N], dist[N];
queue<int> q;
bool vis[N];

int read() {
    int x = 0, f = 1;
    char ch = getchar();
    while (ch < '0' || ch > '9') { f = (ch == '-' ? -1 : f); ch = getchar(); }
    while (ch >= '0' && ch <= '9') { x = x * 10 + ch - '0'; ch = getchar(); }
    return x * f;
}

int find(int x) {
    if (x == fa[x]) return x;
    return fa[x] = find(fa[x]);
}


signed main() {
    n = read(), s = read(), t = read();
    for (int i = 1; i <= n; i++) fa[i] = i;
    for (int i = 1; i < n; i++) {
        int x = read(), y = read(), z = read();
        v[x].push_back((edge){y, z});
        v[y].push_back((edge){x, z});
    }
    vis[s] = vis[t] = true;
    dist[s] = dist[t] = 0;
    q.push(s);
    while(q.size()) {
        int l = q.front(); q.pop();
        for (int i = 0; i < v[l].size(); i++) {
            if (vis[v[l][i].y]) continue;
            q.push(v[l][i].y);
            fa[find(v[l][i].y)] = find(s), vis[v[l][i].y] = true;
            dist[v[l][i].y] = dist[l] + v[l][i].w;
        }
    }
    q.push(t);
    bool flag = false;
    while (q.size()) {
        int l = q.front(); q.pop();
        for (int i = 0; i < v[l].size(); i++) {
            if (find(v[l][i].y) != find(s)) {
                if (vis[v[l][i].y]) continue;
                q.push(v[l][i].y);
                fa[find(v[l][i].y)] = find(t), vis[v[l][i].y] = true;
                dist[v[l][i].y] = dist[l] + v[l][i].w;
            }
            else if (!flag) {
                if (dist[v[l][i].y] > dist[l] + v[l][i].w) {
                    dist[v[l][i].y] = dist[l] + v[l][i].w;
                    fa[v[l][i].y] = find(t);
                }
                flag = true;
            }
        }
    }
    memset(dist, 0, sizeof(dist));
    memset(vis, 0, sizeof(vis));
    int sum = 0;
    q.push(s);
    vis[s] = true;
    while (q.size()) {
        int l = q.front(); q.pop();
        int len = v[l].size();
        for (int i = 0; i < len; i++) {
            if (vis[v[l][i].y]) continue;
            if (fa[find(v[l][i].y)] != find(s)) continue;
            dist[v[l][i].y] = dist[l] + v[l][i].w;
            sum += dist[v[l][i].y], vis[v[l][i].y] = true;
            q.push(v[l][i].y);
        }
    }
    //cout << sum << endl;
    q.push(t), vis[t] = true;
    while (q.size()) {
        int l = q.front(); q.pop();
        for (int i = 0; i < v[l].size(); i++) {
            if (vis[v[l][i].y]) continue;
            dist[v[l][i].y] = dist[l] + v[l][i].w;
            sum += dist[v[l][i].y], vis[v[l][i].y] = true;
            q.push(v[l][i].y);
        }
    }
    cout << sum << endl;
    cout << "IAKIOI";
    return 0;
}

比赛的时候忘记交了，但是大样例过不去。

仅针对第一问