BZOJ 1912 巡逻

重赋边权。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxv 100500
#define maxe 200500
#define inf 2000000000
using namespace std;
int n,k,x,y,g[maxv],nume=1,fath_w[maxv],dis[maxv],pos,ret=0,root=1,tot=0,fath[maxv];
int dp1[maxv],dp2[maxv],ans=-inf;
struct edge
{
     int v,w,nxt;
}e[maxe];
void addedge(int u,int v,int w)
{
    e[++nume].v=v;
    e[nume].w=w;
    e[nume].nxt=g[u];
    g[u]=nume;
}
void dfs1(int x)
{
    for (int i=g[x];i;i=e[i].nxt)
    {
        int v=e[i].v;
        if (v!=fath[x])
        {
            dis[v]=dis[x]+e[i].w;
            fath_w[v]=i;fath[v]=x;
            if (dis[v]>ret)
            {
                ret=dis[v];
                pos=v;
            }
            dfs1(v);
        }
    }
}
int get_d()
{
     dis[root]=0;ret=-inf;memset(fath,0,sizeof(fath));dfs1(root);
     root=pos;
     dis[root]=0;ret=-inf;memset(fath,0,sizeof(fath));dfs1(root);
     return ret;
}
void get_labled(int x)
{
    while (x!=root)
    {
        e[fath_w[x]].w=e[fath_w[x]^1].w=-1;
        x=fath[x];
    }
}
void dfs2(int x)
{
    dp1[x]=dp2[x]=0;
    for (int i=g[x];i;i=e[i].nxt)
    {
        int v=e[i].v;
        if (v!=fath[x])
        {
            dfs2(v);
            if (dp1[v]+e[i].w>dp1[x]) {dp2[x]=dp1[x];dp1[x]=dp1[v]+e[i].w;}
            else if (dp1[v]+e[i].w>dp2[x]) dp2[x]=dp1[v]+e[i].w;
        }
    }
    ans=max(ans,dp1[x]+dp2[x]);
}
int main()
{
    scanf("%d%d",&n,&k);
    for (int i=1;i<=n-1;i++)
    {
        scanf("%d%d",&x,&y);
        addedge(x,y,1);addedge(y,x,1);
    }
    tot+=get_d();
    if (k==1) {printf("%d\n",2*n-1-tot);return 0;}
    get_labled(pos);
    dfs2(root);tot+=ans;
    printf("%d\n",2*n-tot);
    return 0;
}