数据太水， 神奇错误代码竟得91分

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=10000005;
int n, s;
int lft, rgh;
int ans=INT_MAX, maxn, sum[N];
int pre[N];
int path[N], num;
int lng[N];
int dep[N];
bool vis[N];
struct edge{
    int to, w, nex;
} edge[N];
int head[N], tot=1;
void add(int x, int y, int z) {
    edge[++tot]={y, z, head[x]};
    head[x]=tot;
}
queue<int> q;
int dis[N];
int bfs(int s) {
    int e=0, maxn=0;
    memset(dis, -1, sizeof dis);
    q.push(s);
    dis[s]=0;
    while (q.size()) {
        int x=q.front();
        q.pop();
        for (int i=head[x]; i; i=edge[i].nex) {
            int y=edge[i].to, z=edge[i].w;
            if (dis[y]!=-1) continue;
            dis[y]=dis[x]+z, pre[y]=i;
            if (dis[y]>maxn) maxn=dis[y], e=y;
            q.push(y);
        }
    }
    return e;
}
void get() { //yes
    while (lft!=rgh) {
        path[++num]=rgh;
        lng[num+1]=edge[pre[rgh]].w;
        vis[rgh]=1;
        rgh=edge[pre[rgh]^1].to;
    }
    path[++num] = rgh;
    vis[rgh] = 1;
}
void dfs(int x) { //yes
    vis[x]=1;
    for (int i=head[x]; i; i=edge[i].nex) {
        int y=edge[i].to, z=edge[i].w;
        if (vis[y]) continue;
        dfs(y);
        dep[x]=max(dep[x], dep[y]+z);
    }
}
signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);
    cin >> n >> s;
    int x, y, z;
    for (int i=1; i<n; ++i) {
        cin >> x >> y >> z;
        add(x, y, z), add(y, x, z);
    }
    lft=bfs(1); rgh=bfs(lft);
    get();
    for (int i=1; i<=num; ++i) {
        dfs(path[i]);//原来是dfs(i);
        maxn=max(maxn, dep[path[i]]);
        sum[i]=sum[i-1]+lng[i];
    }
    for (int i=1, j=1; i<=num; ++i) {
        while (j<num && sum[j+1]-sum[i]<=s) ++j;
        ans=min(ans, max(max(sum[i], sum[num]-sum[j]), maxn));
    }
    cout << ans << '\n';
    return 0;
}