NOI的。。做一遍dfs就好了？为什么在codevs会RE。。。。
1 #include<bits/stdc++.h> 2 #define inc(i,l,r) for(i=l;i<=r;i++) 3 #define dec(i,l,r) for(i=l;i>=r;i--) 4 #define inf 1e9 5 #define mem(a) memset(a,0,sizeof(a)) 6 #define ll long long 7 #define succ(x) (1<<x) 8 #define NM 2000000 9 using namespace std; 10 int read(){ 11 int x=0,f=1;char ch=getchar(); 12 while(!isdigit(ch)){if(ch=='-')f=-1;ch=getchar();} 13 while(isdigit(ch))x=x*10+ch-'0',ch=getchar(); 14 return x*f; 15 } 16 struct edge{ 17 int t,v; 18 edge *next; 19 }e[2*NM],*h[NM]; 20 int b[NM],n,_x,_y,t,i,s; 21 ll ans; 22 bool v[NM]; 23 void add(int x,int y,int v){ 24 e[++s].t=y;e[s].v=v;e[s].next=h[x];h[x]=&e[s]; 25 } 26 void dfs(int x){ 27 v[x]++; 28 for(edge *j=h[x];j;j=j->next) 29 if(!v[j->t]){ 30 dfs(j->t); 31 b[x]+=b[j->t]; 32 ans+=(ll)j->v*abs(n-2*b[j->t]); 33 } 34 b[x]++; 35 } 36 int main(){ 37 // freopen("test.in","r",stdin); 38 n=read(); 39 inc(i,1,n-1){ 40 _x=read();_y=read();t=read(); 41 add(_x,_y,t);add(_y,_x,t); 42 } 43 dfs(1); 44 printf("%lld ",ans); 45 return 0; 46 }