85分,第14到16点错了,自己看了半天不知道哪里有问题,求助
//#pragma optimize("Ofast")
#include <bits/stdc++.h>
#define MAXN 4000005
#define MAXK 10000007
#define inf 20000007
using namespace std;
typedef long long ll;
int N, K;
struct edge
{
int v;
ll w;
edge(int v= 0, ll w= 0) : v(v), w(w)
{
}
};
vector<edge> adj[MAXN];
int vis[MAXN];
ll d[MAXN];
int sz[MAXN];
int rt, cnt;
int t[MAXK], ans= inf;
int dep[MAXN];
void dfs_rt(int u, int fa, int tot)
{
//++cnt;
sz[u]= 1;
int v, w, n= 0;
for (int k= 0; k < adj[u].size(); k++) {
v= adj[u][k].v;
if (v == fa || vis[v])
continue;
dfs_rt(v, u, tot);
sz[u]+= sz[v];
n= max(n, sz[v]);
}
n= max(n, tot - sz[u]);
if (2 * n <= tot)
rt= u;
}
void dfs1(int u, int fa) //更新答案
{
++cnt;
if (K >= d[u])
ans= min(ans, dep[u] + t[K - d[u]]);
int v, w;
for (int k= 0; k < adj[u].size(); k++) {
v= adj[u][k].v;
w= adj[u][k].w;
if (v == fa || vis[v])
continue;
d[v]= d[u] + w;
dep[v]= dep[u] + 1;
dfs1(v, u);
}
}
void dfs2(int u, int fa, int flag) //更新t
{
if (K >= d[u]) {
if (flag == 1)
t[d[u]]= min(t[d[u]], dep[u]);
else
t[d[u]]= inf;
}
int v;
for (int k= 0; k < adj[u].size(); k++) {
v= adj[u][k].v;
if (v == fa || vis[v])
continue;
dfs2(v, u, flag);
}
}
void work(int u, int fa, int tot)
{
dfs_rt(u, fa, tot);
u= rt;
vis[u]= 1;
d[u]= 0;
t[0]= 1;
int v, w;
for (int k= 0; k < adj[u].size(); k++) {
v= adj[u][k].v;
w= adj[u][k].w;
if (vis[v])
continue;
cnt= 0;
d[v]= w; //路径
dep[v]= 1;
dfs1(v, u);
sz[v]= cnt;
dfs2(v, u, +1);
}
dfs2(u, 0, -1);
for (int k= 0; k < adj[u].size(); k++) {
v= adj[u][k].v;
if (vis[v])
continue;
work(v, u, sz[v]);
}
}
int main()
{
scanf("%d%d", &N, &K);
memset(t, inf, sizeof(t));
int u, v, w;
for (int i= 1; i < N; i++) {
scanf("%d%d%d", &u, &v, &w);
u++;
v++;
adj[u].push_back(edge(v, w));
adj[v].push_back(edge(u, w));
}
memset(t, inf, sizeof t);
t[0]= 1;
work(1, 0, N);
printf("%d\n", ans >= N ? -1 : ans);
return 0;
}