[LUOGU] 4149 [IOI2011]Race

点分治裸题

#include<iostream>
#include<cstring>
#include<cstdio>

using namespace std;

inline int rd(){
    int ret=,f=;char c;
    while(c=getchar(),!isdigit(c))f=c=='-'?-:;
    while(isdigit(c))ret=ret*+c-'',c=getchar();
    return ret*f;
}

const int MAXN=;

struct Edge{
    int next,to,w;
}e[MAXN<<];
int ecnt,head[MAXN];
inline void add(int x,int y,int w){
    e[++ecnt].next = head[x];
    e[ecnt].to = y;
    e[ecnt].w = w;
    head[x] = ecnt;
}

int n,m;

bool vis[MAXN];
int siz[MAXN];
void getsiz(int x,int pre){
    siz[x]=;
    for(int i=head[x];i;i=e[i].next){
        int v=e[i].to;
        if(v==pre||vis[v]) continue;
        getsiz(v,x);
        siz[x]+=siz[v];
    }
}
int mn,root;
void getroot(int x,int pre,int tot){
    int mx=;
    for(int i=head[x];i;i=e[i].next){
        int v=e[i].to;
        if(v==pre||vis[v]) continue;
        mx=max(mx,siz[v]);
        getroot(v,x,tot);
    }
    mx=max(mx,tot-siz[x]);
    if(mx<mn) mn=mx,root=x;
}

int s[MAXN],l[MAXN],sav[MAXN];
int f[],g[];
void dfs(int x,int pre,int dis,int len){
    s[++s[]]=dis;l[++l[]]=len;sav[++sav[]]=dis;
    for(int i=head[x];i;i=e[i].next){
        int v=e[i].to;
        if(vis[v]||v==pre) continue;
        dfs(v,x,dis+e[i].w,len+);
    }
}

int ans=0x3f3f3f3f;
void dac(int x){
    g[]=;f[]=;sav[]=;mn=n;
    getsiz(x,-);
    getroot(x,-,siz[x]);
    int u=root;vis[u]=;
    for(int i=head[u];i;i=e[i].next){
        int v=e[i].to;
        if(vis[v]) continue;
        l[]=;s[]=;dfs(v,x,e[i].w,);
        for(int j=s[];j>=;j--){
            if(s[j]>m) continue;
            if(f[m-s[j]]) ans=min(ans,g[m-s[j]]+l[j]);
        }
        for(int j=s[];j>=;j--){
            if(s[j]>m) continue;//%%%%Monster_Qi
            f[s[j]]=;
            g[s[j]]=min(g[s[j]],l[j]);
        }
    }
    for(int i=sav[];i>=;i--) if(sav[i]<=m) f[sav[i]]=,g[sav[i]]=0x3f3f3f3f;
    for(int i=head[u];i;i=e[i].next){
        int v=e[i].to;
        if(!vis[v]) dac(v);
    }
}

int main(){
    memset(g,0x3f,sizeof(g));
    n=rd();m=rd();
    int x,y,w;
    for(int i=;i<n;i++){
        x=rd();y=rd();w=rd();
        add(x,y,w);add(y,x,w);
    }
    dac();
    if(ans>=0x3f3f3f3f) cout<<-;
    else cout<<ans;
    return ;
}