BZOJ 1912 巡逻（树直径）

struct node
{
    int u,v,w,next;
};


node edges[N<<1];
int head[N],e;


void add(int u,int v,int w)
{
    edges[e].u=u;
    edges[e].v=v;
    edges[e].w=w;
    edges[e].next=head[u];
    head[u]=e++;
}


int n,m,Max;


int son1[N],son2[N],f[N],p;


void DFS(int u,int pre)
{
    int i,v,w,x1=0,x2=0;
    for(i=head[u];i!=-1;i=edges[i].next)
    {
        v=edges[i].v;
        w=edges[i].w;
        if(v==pre) continue;
        DFS(v,u);
        if(f[v]+w>x1) x2=x1,son2[u]=son1[u],son1[u]=i,x1=f[v]+w;
        else if(f[v]+w>x2) x2=f[v]+w,son2[u]=i;
    }
    f[u]=x1;
    if(x1+x2>Max) Max=x1+x2,p=u;
}


int main()
{
    clr(head,-1);
    RD(n,m);
    int i,u,v;
    FOR1(i,n-1)
    {
        RD(u,v);
        add(u,v,1);
        add(v,u,1);
    }
    int ans=(n-1)<<1;
    clr(son1,-1); clr(son2,-1);
    Max=0; DFS(1,-1); ans-=Max-1;
    for(i=son1[p];i!=-1;i=son1[edges[i].v]) edges[i].w=edges[i^1].w=-1; 
    for(i=son2[p];i!=-1;i=son1[edges[i].v]) edges[i].w=edges[i^1].w=-1;
    clr(son1,-1); clr(son2,-1);
    if(m>1) Max=0,DFS(1,-1),ans-=Max-1;
    PR(ans);
}