点分治板子而已

每次算一下到目前树根有多少条边就是了，取min

/*
    有一个条件,就是要求边的数量最小,那么可以考虑,因为我们在模板中判断是否为k时时用的bool,我们可以开一个int.
    它存下的值就是最短边数.
*/
#include<bits/stdc++.h>
using namespace std;
const int N=1e6+50;
const int M=2e6+50;
int rt;
int sum;
int res;
int n,k;
int tot;
int cnt;
int s[N];
int x,y,z;
bool vis[N];
int head[N];
int judge[M];
int tmp[N],dis[N];
int siz[N],maxx[N];
struct Edge {
    int u,v,w;
} edge[N*3];
void add(int x,int y,int z) {
    edge[++tot].u=head[x];
    edge[tot].v=y;
    edge[tot].w=z;
    head[x]=tot;
    return ;
}
void getrt(int x,int f) {
    siz[x]=1;
    maxx[x]=0;
    for(int i=head[x]; i; i=edge[i].u) {
        int to=edge[i].v;
        if(to==f||vis[to]) continue;
        getrt(to,x);
        siz[x]+=siz[to];
        maxx[x]=max(maxx[x],siz[to]);
    }
    maxx[x]=max(maxx[x],sum-siz[x]);
    if(maxx[x]<maxx[rt]) rt=x;
    return ;
}
void getdis(int x,int f,int bian) {
    if(dis[x]>1e6) return ;
    tmp[++cnt]=dis[x];
    s[cnt]=bian;
    for(int i=head[x]; i; i=edge[i].u) {
        int to=edge[i].v;
        if(to==f||vis[to]) continue;
        dis[to]=dis[x]+edge[i].w;
        getdis(to,x,bian+1);
    }
    return ;
}
void solve(int x) {
    queue<int> que;
    for(int i=head[x]; i; i=edge[i].u) {
        int to=edge[i].v;
        if(vis[to]) continue;
        cnt=0;
        dis[to]=edge[i].w;
        getdis(to,x,1);
        for(int j=1; j<=cnt; ++j)
            if(k>=tmp[j])
                res=min(res,s[j]+judge[k-tmp[j]]);
        for(int j=1; j<=cnt; ++j) {
            que.push(tmp[j]);
            judge[tmp[j]]=min(judge[tmp[j]],s[j]);
        }
    }
    while(!que.empty()) {
        judge[que.front()]=0x3f3f3f3f;
        que.pop();
    }
    return ;
}
void divide(int x) {
    vis[x]=true;
    judge[0]=0;
    solve(x);
    for(int i=head[x]; i; i=edge[i].u) {
        int to=edge[i].v;
        if(vis[to]) continue;
        maxx[rt=0]=sum=siz[to];
        getrt(to,0);
        getrt(rt,0);
        divide(rt);
    }
    return ;
}
int main() {
//  freopen("race.in","r",stdin);
//  freopen("race.out","w",stdout);
    res=0x3f3f3f3f;
    memset(judge,0x3f,sizeof judge);
    scanf("%d%d",&n,&k);
    for(int i=1; i<n; ++i) {
        scanf("%d%d%d",&x,&y,&z);
        x++;
        y++;
        add(x,y,z);
        add(y,x,z);
    }
    maxx[0]=sum=n;
    getrt(1,0);
    getrt(rt,0);
    divide(rt);
    if(res==0x3f3f3f3f) printf("-1\n");
    else printf("%d\n",res);
    return 0;
}