Luogu P1131 [ZJOI2007]时态同步 树形DP

#include<cstdio>
#include<iostream>
#include<cstring>
#define R register int
using namespace std;
struct edge {
    int v,w,nxt;
}e[1000010];
int n,cnt,s;
int fir[500010];
long long ans,mx[500010];
inline void add(int u,int v,int w) {e[++cnt].v=v,e[cnt].w=w,e[cnt].nxt=fir[u],fir[u]=cnt;}
inline int g() {
    R ret=0,fix=1; register char ch; while(!isdigit(ch=getchar())) fix=ch==‘-‘?-1:fix;
    do ret=ret*10+(ch^48); while(isdigit(ch=getchar())); return ret*fix;
}
void dfs(int u,int fa) {
    //cout<<u<<" "<<fa<<endl;
    for(R i=fir[u];i;i=e[i].nxt) {
        R v=e[i].v;
        if(v==fa) continue;
        dfs(v,u); mx[u]=max(mx[u],mx[v]+e[i].w);
    }
    for(R i=fir[u];i;i=e[i].nxt) {
        R v=e[i].v;
        if(v==fa) continue;
        ans+=mx[u]-(mx[v]+e[i].w);
    }
}
signed main() {
    n=g(),s=g();
    for(R i=1;i<n;++i){R u=g(),v=g(),w=g(); add(u,v,w),add(v,u,w);}
    dfs(s,0); printf("%lld\n",ans);
}