luogu 3441 [POI2006]MET-Subway 拓扑排序+思维

Code:

// luogu-judger-enable-o2
#include <queue> 
#include <cstdio>  
#include <algorithm>  
#define N 1000005 
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std;   
queue<int>q;   
int n,edges; 
int deg[N],hd[N],to[N<<1],nex[N<<1],num[N],dep[N],vis[N]; 
void add(int u,int v) 
{ 
    nex[++edges]=hd[u],hd[u]=edges,to[edges]=v; 
}   
int main() 
{ 
    int i,j,L;   
    // setIO("input");   
    scanf("%d%d",&n,&L); 
    for(i=1;i<n;++i) 
    {
        int a,b; 
        scanf("%d%d",&a,&b),add(a,b),add(b,a),++deg[a],++deg[b]; 
    }     
    for(i=1;i<=n;++i) if(deg[i]==1) q.push(i),vis[i]=1;      
    while(!q.empty()) 
    { 
        int u=q.front();q.pop();   
        for(int i=hd[u];i;i=nex[i]) 
        { 
            if(!vis[to[i]]) 
            {
                --deg[to[i]]; 
                if(deg[to[i]]<=1)
                    vis[to[i]]=1,dep[to[i]]=dep[u]+1,q.push(to[i]);    
            }
        }
    }   
    for(i=1;i<=n;++i) ++num[dep[i]]; 
    int ans=0; 
    for(i=0;i<=n;++i)  
        ans+=min(2*L,num[i]);  
    printf("%d\n",ans); 
    return 0; 
}