luogu 2052 [NOI2011]道路修建 BFS序

据说dfs会爆栈，写一个 BFS 序更新就好了~ 

#include <bits/stdc++.h>
#define N 1000005 
#define ll long long  
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std;
queue<int>q;    
int edges,hd[N],to[N<<1],nex[N<<1],val[N<<1],fa[N],dfn[N],vis[N],size[N],cc[N];    
void add(int u,int v,int c) 
{
    nex[++edges]=hd[u],hd[u]=edges,to[edges]=v,val[edges]=c; 
} 
int main() 
{
    // setIO("input"); 
    int n,i,j,cnt=0; 
    scanf("%d",&n); 
    for(i=1;i<n;++i) 
    {
        int a,b,c; 
        scanf("%d%d%d",&a,&b,&c);   
        add(a,b,c),add(b,a,c); 
    }    
    dfn[++cnt]=1;
    vis[1]=1;     
    q.push(1); 
    for(;!q.empty();) 
    {
        int u=q.front();q.pop();      
        size[u]=1; 
        for(int i=hd[u];i;i=nex[i]) 
        {
            int v=to[i]; 
            if(vis[v]) continue; 
            vis[v]=1;    
            dfn[++cnt]=v;           
            q.push(v);                   
            fa[v]=u;    
            cc[v]=val[i];   
        }
    }     
    ll ans=0; 
    for(int i=cnt;i>=2;--i) 
    {
        int v=dfn[i];   
        int u=fa[v]; 
        size[u]+=size[v];   
        ans+=1ll*abs(n-size[v]-size[v])*cc[v];      
    } 
    printf("%lld\n",ans); 
    return 0; 
}