贪心
贪心思路是先找到每个节点的到最深处的路径，并找到最大值。然后最后答案要加上该最大值和所有路径权值的差。
#include <bits/stdc++.h>
#define N 600101
#define int long long
using namespace std;
int n, root, cnt, ans, lin[N], siz[N], dp[N], maxn[N];//dp[i]表示i为根的树的所需要的时间
struct edg {
int to, nex, len;//len表示的是路径权值，并不只是一条边
}e[N * 3];
inline void add(int f, int t, int l)
{
e[++cnt].to = t;
e[cnt].len = l;
e[cnt].nex = lin[f];
lin[f] = cnt;
}
int dfs(int now, int fa)
{
for (int i = lin[now]; i; i = e[i].nex)
{
int to = e[i].to;
if (to == fa) continue;
dfs(to, now);
e[i].len += maxn[to];
maxn[now] = max(maxn[now], e[i].len);
}
for (int i = lin[now]; i; i = e[i].nex)
{
int to = e[i].to;
if (to == fa)
continue;
ans += maxn[now] - e[i].len;
}
}
signed main()
{
scanf("%lld%lld", &n, &root);
for (int i = 1, a, b, c; i < n; i++)
scanf("%lld%lld%lld", &a, &b, &c), add(a, b, c), add(b, a, c);
dfs(root, 0);
printf("%lld", ans);
return 0;
}
/*
4 1
1 2 1
1 3 3
3 4 5
*/