BZOJ 1060: [ZJOI2007]时态同步 树上问题_贪心

#include <bits/stdc++.h>
#define setIO(s) freopen(s".in","r",stdin)
#define maxn 500002
#define inf 1000000000000000
#define ll long long 
using namespace std;
ll dis[maxn], s[maxn]; 
int hd[maxn], to[maxn << 1], nex[maxn << 1], val[maxn << 1], fa[maxn], chil[maxn];    
int edges, n, root; 
ll maxv, ans;  
void add(int u, int v, int c)
{
    nex[++edges] = hd[u], hd[u] = edges, to[edges] = v, val[edges] = c; 
}
void dfs(int u, int ff)
{
    fa[u] = ff, chil[u] = 0; 
    for(int i = hd[u]; i ; i = nex[i])
    {
        int v = to[i]; 
        if(to[i] == ff) continue; 
        ++chil[u]; 
        dis[v] = dis[u] + (ll) val[i]; 
        dfs(v, u); 
    }    
} 
void dfs2(int u)
{ 
    if(chil[u] == 0) 
    {
        s[u] = maxv - dis[u]; 
        return ; 
    }
    ll _min = inf; 
    for(int i = hd[u]; i ; i = nex[i])
    {
        int v = to[i]; 
        if(v == fa[u]) continue; 
        dfs2(v); 
        _min = min(_min, s[v]);       
    }
    s[u] = _min; 
    for(int i = hd[u]; i ; i = nex[i]) 
    {
        int v = to[i]; 
        if(v == fa[u]) continue; 
        ans += s[v] - _min; 
    }     
}
int main()
{
   //  setIO("input");  
    scanf("%d%d",&n,&root); 
    for(int i = 1, u, v, c; i < n ; ++i)
    {
        scanf("%d%d%d",&u,&v,&c); 
        add(u, v, c), add(v, u, c); 
    }
    dfs(root, 0);      
    for(int i = 1; i <= n ; ++i) maxv = max(maxv, dis[i]);   
    dfs2(root);  
    printf("%lld\n",ans); 
    return 0; 
}