luogu1131 [ZJOI2007][时态同步]

#include <vector>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
#define LL long long
const int N = 5e5 + 100 ;
struct Edge{
int v , w ;
Edge ( int x=0 , int y=0 ) : v(x) , w(y) {}
};
vector <Edge> edges ;
vector <int> G
;

void addeage ( int u , int v , int w ) {
edges.push_back ( Edge ( v , w ) ) ;
int tot = edges.size () ;
G [u].push_back ( tot-1 ) ;
}

int f
, vis
;

void dfs ( int u ) {
vis [u] = 1 ;
for ( int i = 0 ; i < G [u].size () ; ++ i ) {
Edge e = edges [ G [u] [i] ] ;
if ( vis [e.v] ) continue ;
dfs ( e.v ) ;
f [u] = max ( f [u] , f [e.v] + e.w ) ;
}
}
LL ans = 0 ;
void dfa ( int u ) {
vis [u] = 1 ;
for ( int i = 0 ; i < G [u].size () ; ++ i ) {
Edge e = edges [ G [u] [i] ] ;
if ( vis [e.v] ) continue ;

dfa ( e.v ) ;
ans += f [u] - ( f [e.v] + e.w ) ;
}
}

int main () {
int n , root ;
scanf ( "%d%d" , &n , &root ) ;
for ( int i = 1 ; i < n ; ++ i ) {
int x , y , z ;
scanf ( "%d%d%d" , &x , &y , &z ) ;
addeage ( x , y , z ) ;
addeage ( y , x , z ) ;
}
dfs ( root );
memset ( vis , 0 , sizeof vis ) ;
dfa ( root );
printf ( "%lld" , ans );
return 0 ;
}