本来以为是一道傻逼题，然而跪了好久……一直RE……
直接dfs就好了……x->y val=c : ans+=abs(n-size[y]-size[y])*c;
然而为啥会一直RE呢？
后来改成dfs返回当前节点的size就过了……过了……
100W的节点难道linux下也会爆栈嘛= =？sad
1 /************************************************************** 2 Problem: 2435 3 User: Tunix 4 Language: C++ 5 Result: Accepted 6 Time:5292 ms 7 Memory:73348 kb 8 ****************************************************************/ 9 10 //BZOJ 2435 11 #include<vector> 12 #include<cstdio> 13 #include<cstring> 14 #include<cstdlib> 15 #include<iostream> 16 #include<algorithm> 17 #define rep(i,n) for(int i=0;i<n;++i) 18 #define F(i,j,n) for(int i=j;i<=n;++i) 19 #define D(i,j,n) for(int i=j;i>=n;--i) 20 #define pb push_back 21 using namespace std; 22 inline int getint(){ 23 int v=0,sign=1; char ch=getchar(); 24 while(ch<'0'||ch>'9'){ if (ch=='-') sign=-1; ch=getchar();} 25 while(ch>='0'&&ch<='9'){ v=v*10+ch-'0'; ch=getchar();} 26 return v*sign; 27 } 28 const int N=1001000,INF=~0u>>2; 29 typedef long long LL; 30 /******************tamplate*********************/ 31 32 int to[N<<1],nxt[N<<1],head[N],cnt,l[N<<1]; 33 void add(int x,int y,int z){ 34 to[++cnt]=y; nxt[cnt]=head[x]; head[x]=cnt; l[cnt]=z; 35 to[++cnt]=x; nxt[cnt]=head[y]; head[y]=cnt; l[cnt]=z; 36 } 37 int n,s[N]; 38 LL ans; 39 40 int dfs(int x,int fa){ 41 s[x]=1; 42 for(int i=head[x];i;i=nxt[i]) 43 if (to[i]!=fa){ 44 s[x]+=dfs(to[i],x); 45 ans+=(LL)abs(n-s[to[i]]*2)*l[i]; 46 } 47 return s[x]; 48 } 49 int main(){ 50 #ifndef ONLINE_JUDGE 51 freopen("2435.in","r",stdin); 52 freopen("2435.out","w",stdout); 53 #endif 54 n=getint(); 55 F(i,2,n){ 56 int x=getint(),y=getint(),z=getint(); 57 add(x,y,z); 58 } 59 dfs(1,0); 60 cout <<ans<<endl; 61 return 0; 62 }