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;
}