luogu1131 [ZJOI2007][时态同步]

树形DP
f [i］表示 i 的子树更新上来的最大值
f [i] = max ( f [i] , f [v] + w )
ans += f [i] - f [v] - w

#include <vector>#include <cstdio>#include <cstring>#include <iostream>#include <algorithm>using namespace std;#define LL long longconst 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 [N] ;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 [N] , vis [N] ;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 ;}