树分治（挑战p360）

Posted 2022-11-17 starve

poj1741

题：http://poj.org/problem?id=1741

#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
using namespace std;
typedef long long ll;
const int inf=0x3f3f3f3f;
const ll INF=1e18;
int ans=0,tot,maxx,root;
const int M=1e4+4;
struct node
    int v,w,nextt;
e[M<<1];
int head[M],vis[M],sz[M],maxv[M],num,dis[M],n,k;
void addedge(int u,int v,int w)
    e[tot].v=v;
    e[tot].w=w;
    e[tot].nextt=head[u];
    head[u]=tot++;

void dfssz(int u,int f)
    sz[u]=1;
    maxv[u]=0;
    for(int i=head[u];~i;i=e[i].nextt)
        int v=e[i].v;//cout<<"!!"<<endl;
        if(v==f||vis[v])
            continue;
        dfssz(v,u);
        sz[u]+=sz[v];
        maxv[u]=max(maxv[u],sz[v]);
    

void dfsroot(int r,int u,int f)
    maxv[u]=max(maxv[u],sz[r]-sz[u]);
    if(maxv[u]<maxx)
        maxx=maxv[u];
        root=u;
    
    for(int i=head[u];~i;i=e[i].nextt)
        int v=e[i].v;
        if(vis[v]||v==f)
            continue;
        dfsroot(r,v,u);
    

void dfsdis(int u,int d,int f)
    dis[num++]=d;
    for(int i=head[u];~i;i=e[i].nextt)
        int v=e[i].v;
        if(v==f||vis[v])
            continue;
        dfsdis(v,d+e[i].w,u);
    

int cal(int u,int d)
    int sum=0;
    num=0;
    dfsdis(u,d,-1);
    sort(dis,dis+num);
    int i=0,j=num-1;
    while(i<j)
        while(dis[i]+dis[j]>k&&i<j)
            j--;
        
        sum+=j-i;
        i++;
    
    return sum;

void solve(int u)
    maxx=n;
    dfssz(u,-1);
    dfsroot(u,u,-1);
    ans+=cal(root,0);
    
    vis[root]=1;
    for(int i=head[root];~i;i=e[i].nextt)
        int v=e[i].v;
        if(vis[v])
            continue;
        ans-=cal(v,e[i].w);
        solve(v);
    

int main()
    while(~scanf("%d%d",&n,&k)&&(n+k))
        tot=0,ans=0;
        memset(head,-1,sizeof(head));
        memset(vis,0,sizeof(vis));
        for(int i=1;i<n;i++)
            int u,v,w;
            scanf("%d%d%d",&u,&v,&w);
            addedge(u,v,w);
            addedge(v,u,w);
        
        
        solve(1);
        printf("%lld\n",ans);
    
    return 0;

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