bzoj1060 [ZJOI2007]时态同步

#include<bits/stdc++.h>
#define N 500000
using namespace std;
int n,rt,x,y,z;
int first[N+1],nex[2*N+1],to[2*N+1],val[2*N+1],siz;
long long f[N+1],ans;
int read()
{
int x=0;
char c=getchar();
while(c<'0'||c>'9')c=getchar();
while(c<='9'&&c>='0')x=x*10+c-'0',c=getchar();
return x;
}
void add(int x,int y,int z)
{
nex[siz]=first[x];
first[x]=siz;
to[siz]=y;
val[siz]=z;
siz++;
}
void DFS(int x,int fa)
{
f[x]=0;
for(int i=first[x];i!=-1;i=nex[i])
{
int u=to[i];
if(u==fa)continue;
DFS(u,x);
f[x]=max(f[x],f[u]+val[i]);
}
for(int i=first[x];i!=-1;i=nex[i])
if(to[i]!=fa)ans+=f[x]-f[to[i]]-val[i];
}
int main()
{
//freopen("in.txt","r",stdin);
memset(first,-1,sizeof(first));
n=read(),rt=read();
for(int i=1;i<n;i++)
{
x=read(),y=read(),z=read();
add(x,y,z),add(y,x,z);
}
DFS(rt,0);
cout<<ans;
return 0;
}