[IOI2011]Race $O(nlog^{2}n)$ 做法

这个应该还不是正解，明天看一下正解到底是什么...

Code:

#include<bits/stdc++.h>
#define setIO(s) freopen(s".in","r",stdin)
#define maxn 500000
#define inf 0x7f7f7f
using namespace std; 
char *q1,*q2,buf[1000000];
#define nc() (q1==q2&&(q2=(q1=buf)+fread(buf,1,1000000,stdin),q1==q2)?EOF:*q1++)
int rd() {int x=0; char c=nc(); while(c<48) c=nc(); while(c>47) x=(((x<<2)+x)<<1)+(c^48),c=nc(); return x;}
int hd[maxn],to[maxn],nex[maxn],val[maxn],f[maxn],vis[maxn],siz[maxn],dep[maxn],ans,cur=0;
int cnt,n,k,root,sn,d[2];
void addedge(int u,int v,int c)
{
    nex[++cnt] = hd[u],hd[u] = cnt,to[cnt] = v,val[cnt] = c;
}
struct Point{
    int dis,tot;
    Point(int dis = 0,int tot = 0):dis(dis),tot(tot){}
    bool operator<(Point b)const{
        return b.dis != dis ? b.dis>dis : b.tot > tot; 
    }
}point[2][maxn],pp[maxn]; 
void getdis(int u,int fa,int tot)
{
    if(!u) return; 
    point[cur][++d[cur]]=Point(dep[u],tot);
    for(int i = hd[u]; i ; i = nex[i])
    {
        if(vis[to[i]] || to[i] == fa) continue; 
        dep[to[i]] = dep[u] + val[i],getdis(to[i],u,tot + 1);
    }    
}
void GetRoot(int u,int ff)
{
    siz[u] = 1,f[u] = 0;      
    for(int i = hd[u]; i ; i = nex[i])
    {
        if(vis[to[i]] || to[i] == ff) continue;   
        GetRoot(to[i],u);
        siz[u] += siz[to[i]]; 
        f[u] = max(f[u], siz[to[i]]);
    }
    f[u] = max(f[u], sn - siz[u]);
    if(f[u] < f[root]) root = u;
} 
void DFS(int u)
{
    //calc
    d[cur^1]=d[cur]=0,vis[u]=1; 
    point[cur^1][++d[cur^1]]=Point(0,0); 
    for(int i = hd[u],j,m,p1,p2; i ; i = nex[i])
    {

        if(vis[to[i]]) continue; 
        dep[to[i]]=val[i]; 
        getdis(to[i],u,1);     
        sort(point[cur]+1,point[cur]+1+d[cur]);
        m = 1; 
        for(j = 2;j <= d[cur]; ++j) 
            if(point[cur][j].dis!=point[cur][j-1].dis) point[cur][++m]=point[cur][j];     
        d[cur] = m,p1 = 1,p2 = d[cur^1];               
        while(p1 <= d[cur] && p2 >=1)
        {        
            if(point[cur][p1].dis + point[cur^1][p2].dis == k) 
                ans = min(ans,point[cur][p1].tot + point[cur^1][p2].tot), ++p1,--p2;
            if(point[cur][p1].dis + point[cur^1][p2].dis < k) ++p1;
            if(point[cur][p1].dis + point[cur^1][p2].dis > k) --p2;
        }
        for(j = 1;j <= d[cur]; ++j) point[cur^1][++d[cur^1]] = point[cur][j];
        sort(point[cur^1]+1,point[cur^1]+1+d[cur^1]);   
        m = 1; 
        for(j=2;j<=d[cur^1];++j) 
            if(point[cur^1][j].dis!=point[cur^1][j-1].dis)point[cur^1][++m]=point[cur^1][j];         
        d[cur^1]=m, d[cur]=0;  
    }
    for(int i = hd[u]; i ; i = nex[i])
    {
        if(vis[to[i]]) continue; 
        root = 0,sn = siz[to[i]],GetRoot(to[i],u), DFS(root);         
    }
}    
int main()
{
    
    // setIO("input");        
    n=rd(),k=rd(); 
    for(int i = 1,a,b,c;i < n; ++i) a=rd(),b=rd(),c=rd(),addedge(a+1,b+1,c),addedge(b+1,a+1,c);
    ans = inf, vis[0] = 1,f[0] = inf,sn = n,root = 0, GetRoot(1,0); 
    DFS(root);
    printf("%d\n",ans==inf?-1:ans); 
    return 0;
}