MLE求调，悬两关

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 300010, M = N * 2;
int n, m;
int h[N], e[M], w[M], ne[M], idx;
int fa[N][20], dep[N], dis[N];
int seq[N], ts;
int cnt[N];
struct Path{
    int a, b, p, d;
} q[N];
void add(int a, int b, int c) {
    e[idx] = b, w[idx] = c, ne[idx] = h[a], h[a] = idx++;
}
void dfs(int u, int father, int depth) {
    seq[ts++] = u;
    dep[u] = depth;
    for (int i = h[u]; ~i; i = ne[i]) {
        int j = e[i];
        if (j == father) continue;
        fa[j][0] = u;
        for (int k = 1; k < 20; k++) fa[j][k] = fa[fa[j][k - 1]][k - 1];
        dis[j] = dis[u] + w[i];
        dfs(j, u, depth + 1);
    }
}
int lca(int a, int b) {
    if (dep[a] < dep[b]) swap(a, b);
    for (int k = 19; ~k; k--) 
        if (dep[fa[a][k]] >= dep[b]) 
            a = fa[a][k];
    if (a == b) return a;
    for (int k = 19; ~k; k--) 
        if (fa[a][k] != fa[b][k]) 
            a = fa[a][k], b = fa[b][k];
    return fa[a][0];
}
bool check(int x) {
    memset(cnt, 0, sizeof cnt);
    int c = 0, maxd = 0;
    for (int i = 1; i <= m; i++) {
        int a = q[i].a, b = q[i].b, p = q[i].p, d = q[i].d;
        if (d > x) {
            c++;
            maxd = max(maxd, d);
            cnt[a]++, cnt[b]++, cnt[p] -= 2;
        }
    }
    if (!c) return 1;
    for (int i = n - 1; ~i; i--) {
        int j = seq[i];
        cnt[fa[j][0]] += cnt[j];
    }
    for (int i = 1; i <= n; i++) 
        if (c == cnt[i] && maxd - (dis[i] - dis[fa[i][0]]) <= x) 
            return 1;
    return 0;
}
int main() {
    memset(h, -1, sizeof h);
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= n; i++) {
        int a, b, c;
        scanf("%d%d%d", &a, &b, &c);
        add(a, b, c), add(b, a, c);
    }
    dfs(1, -1, 1);
    for (int i = 1; i <= m; i++) {
        int a, b;
        scanf("%d%d", &a, &b);
        int p = lca(a, b);
        int d = dis[a] + dis[b] - dis[p] * 2;
        q[i] = {a, b, p, d};
    }
    int l = 0, r = 3e8;
    while (l < r) {
        int mid = l + r >> 1;
        if (check(mid)) r = mid;
        else l = mid + 1;
    }
    printf("%d\n", r);
    return 0;
}