[BZOJ1999][codevs1167][Noip2007]Core树网的核

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <stack>
#include <vector>
#include <queue>
#include <cstring>
#include <string>
#include <map>
#include <set>
using namespace std;
#define LL long long

LL read() {
    LL x = 0, f = 1; char c = getchar();
    while(!isdigit(c)){ if(c == ‘-‘) f = -1; c = getchar(); }
    while(isdigit(c)){ x = x * 10 + c - ‘0‘; c = getchar(); }
    return x * f;
}

#define maxn 500010
#define maxm 1000010
#define maxlog 20
int n, m, far, head[maxn], next[maxm], to[maxm];
LL dist[maxm];

void AddEdge(int a, int b, LL c) {
	to[++m] = b; dist[m] = c; next[m] = head[a]; head[a] = m;
	swap(a, b);
	to[++m] = b; dist[m] = c; next[m] = head[a]; head[a] = m;
	return ;
}

int Q[maxn], hd, tl;
LL d[maxn];
void BFS(int s) {
	memset(d, -1, sizeof(d));
	hd = tl = 0; Q[++tl] = s;
	d[s] = 0;
	while(hd < tl) {
		int u = Q[++hd];
		for(int e = head[u]; e; e = next[e]) if(d[to[e]] < 0) {
			d[to[e]] = d[u] + dist[e];
			Q[++tl] = to[e];
		}
	}
	return ;
}

int mid1, mid2;
LL md1, md2, td;
bool done[maxn];
void getmid(int s) {
	d[s] = 0; done[s] = 1;
	hd = tl = 0; Q[++tl] = s;
	while(hd < tl) {
		int u = Q[++hd];
		for(int e = head[u]; e; e = next[e]) if(!done[to[e]]) {
			if(d[to[e]] > d[u] + dist[e]) {
				d[to[e]] = d[u] + dist[e];
				Q[++tl] = to[e]; done[to[e]] = 1;
			}
			else if(d[to[e]] == d[u] + dist[e]) {
				mid1 = to[e]; md1 = d[to[e]];
				return ;
			}
			else {
				mid1 = u; mid2 = to[e];
				md1 = d[u]; md2 = d[to[e]];
				td = dist[e];
				return ;
			}
		}
	}
	return ;
}

LL Dep[maxn];
int dep[maxn], vis[maxn], fa[maxn];
void build(int u, int pa) {
	fa[u] = pa;
	for(int e = head[u]; e; e = next[e]) if(to[e] != pa) {
		dep[to[e]] = dep[u] + 1;
		Dep[to[e]] = Dep[u] + dist[e];
		build(to[e], u);
	}
	return ;
}

void BFS2() {
	memset(d, -1, sizeof(d));
	hd = tl = 0;
	for(int i = 1; i <= n; i++) if(vis[i] == 4) Q[++tl] = i, d[i] = 0;
	while(hd < tl) {
		int u = Q[++hd];
		for(int e = head[u]; e; e = next[e]) if(d[to[e]] < 0) {
			d[to[e]] = d[u] + dist[e];
			Q[++tl] = to[e];
		}
	}
	return ;
}

int main() {
	n = read(); far = read();
	for(int i = 1; i < n; i++) {
		int a = read(), b = read(), c = read();
		AddEdge(a, b, c);
	}
	
	BFS(1);
	int cnt = 0, A, B;
	LL mx = 0;
	bool tp = 1;
	for(int i = 1; i <= n; i++) mx = max(mx, d[i]);
	for(int i = 1; i <= n; i++) if(mx == d[i]) cnt++, B = i;
	tp &= (cnt == 1);
	BFS(B);
	mx = cnt = 0;
	for(int i = 1; i <= n; i++) mx = max(mx, d[i]);
	for(int i = 1; i <= n; i++) if(mx == d[i]) cnt++, A = i;
	tp &= (cnt == 1);
	
	getmid(A);
//	printf("%lld %d %d %d %d %lld %lld %lld (%lld %lld)\n", mx, mid1, mid2, A, B, md1, md2, td, max(md1, md2 + td), max(md2, md1 + td));
	if(max(md1, md2 + td) > max(md2, md1 + td)) swap(md1, md2), swap(mid1, mid2);
	build(mid1, 0); vis[mid1] = 4;
//	printf("mid1: %d %lld %lld\n", mid1, Dist(mid1, A), Dist(mid2, B));
	int tmp = A;
	while(tmp != mid1) vis[tmp] = 2, tmp = fa[tmp];
	tmp = B;
	while(tmp != mid1) vis[tmp] = 3, tmp = fa[tmp];
	int a, b; a = b = mid1;
	while(far) {
		int ta = mid1, tb = mid1;
		LL da, db;
		for(int e = head[a]; e; e = next[e]) if(vis[to[e]] == 2) {
			ta = to[e], da = dist[e]; break;
		}
		for(int e = head[b]; e; e = next[e]) if(vis[to[e]] == 3) {
			tb = to[e], db = dist[e]; break;
		}
		if(Dep[A] - Dep[a] > Dep[B] - Dep[b]) {
			if(da <= far) a = ta, vis[a] = 4, far -= da;
			else break;
		}
		else {
			if(db <= far) b = tb, vis[b] = 4, far -= db;
			else break;
		}
	}
	LL ans = 0;
//	printf("%d %d %lld\n", a, b, Dist(a, b));
	BFS2();
	for(int i = 1; i <= n; i++) ans = max(ans, d[i]);
	
	printf("%lld\n", ans);
	
	return 0;
}