bzoj2435: [Noi2011]道路修建

树形dp，bfs。 dfs爆栈。

#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
const int maxn = 1000000 + 10;
const int maxm = 2000000 + 10;
int g[maxn],v[maxm],next[maxm],c[maxm],eid;
int q[maxn],f[maxn],t[maxn],size[maxn];
int n;
long long res;

void addedge(int a,int b,int C) {
    v[eid]=b; next[eid]=g[a]; c[eid]=C; g[a]=eid++;
    v[eid]=a; next[eid]=g[b]; c[eid]=C; g[b]=eid++;
}

void bfs() {
    int l=1,r=1;
    q[r++]=1;
    while(l<r) {
        int u=q[l++];
        size[u]=1;
        for(int i=g[u];~i;i=next[i]) if(v[i]!=f[u]) {
            f[v[i]]=u;
            t[v[i]]=c[i];
            q[r++]=v[i];
        }
    }
}

int ab(int a,int b) {
    int res=a-b;
    if(res<0) return -res;
    else return res;    
}

int main() {
    scanf("%d",&n);
    memset(g,-1,sizeof(g));
    for(int i=1,u,v,C;i<=n-1;i++) {
        scanf("%d%d%d",&u,&v,&C);
        addedge(u,v,C);
    }
    bfs();
    for(int i=n,u;i>1;i--) {
        u=q[i];
        size[f[u]]+=size[u];
        res+=(long long)t[u]*ab(size[u],n-size[u]);
    }
    printf("%lld\n",res);
    return 0;
}