类似于找重心的写法，往牛多的那一头走(更优)，直到找不到更优的
1 #include<algorithm> 2 #include<iostream> 3 #include<cstdlib> 4 #include<cstring> 5 #include<cstdio> 6 #include<string> 7 #include<cmath> 8 #include<ctime> 9 #include<queue> 10 #include<stack> 11 #include<map> 12 #include<set> 13 #define rre(i,r,l) for(int i=(r);i>=(l);i--) 14 #define re(i,l,r) for(int i=(l);i<=(r);i++) 15 #define Clear(a,b) memset(a,b,sizeof(a)) 16 #define inout(x) printf("%d",(x)) 17 #define douin(x) scanf("%lf",&x) 18 #define strin(x) scanf("%s",(x)) 19 #define LLin(x) scanf("%lld",&x) 20 #define op operator 21 #define CSC main 22 typedef unsigned long long ULL; 23 typedef const int cint; 24 typedef long long LL; 25 using namespace std; 26 void inin(int &ret) 27 { 28 ret=0;int f=0;char ch=getchar(); 29 while(ch<'0'||ch>'9'){if(ch=='-')f=1;ch=getchar();} 30 while(ch>='0'&&ch<='9')ret*=10,ret+=ch-'0',ch=getchar(); 31 ret=f?-ret:ret; 32 } 33 int head[100010],next[200020],zhi[200020],ed; 34 LL c[100010],s[100010],dis[100010],w[200020]; 35 LL sum; 36 void add(int a,int b,LL cc) 37 { 38 next[++ed]=head[a],head[a]=ed,zhi[ed]=b,w[ed]=cc; 39 next[++ed]=head[b],head[b]=ed,zhi[ed]=a,w[ed]=cc; 40 } 41 int n; 42 LL ans; 43 void dfs(int x,int fa) 44 { 45 ans+=c[x]*dis[x]; 46 s[x]=c[x]; 47 for(int i=head[x];i;i=next[i])if(zhi[i]!=fa) 48 dis[zhi[i]]=dis[x]+w[i],dfs(zhi[i],x),s[x]+=s[zhi[i]]; 49 } 50 void getroot(int x,int fa) 51 { 52 for(int i=head[x];i;i=next[i])if(zhi[i]!=fa) 53 if((s[zhi[i]]<<1)>sum){ans-=(LL)((s[zhi[i]]<<1)-sum)*w[i],getroot(zhi[i],x);break;} 54 } 55 int main() 56 { 57 inin(n); 58 re(i,1,n)LLin(c[i]),sum+=c[i]; 59 re(i,2,n) 60 { 61 int a,b,cc; 62 inin(a),inin(b),inin(cc); 63 add(a,b,cc); 64 } 65 dfs(1,0); 66 getroot(1,0); 67 printf("%lld",ans); 68 return 0; 69 }