P4568 [JLOI2011] 飞行路线 分层图板子，调了两年半了...

#include <bits/stdc++.h>
#define re register int
#define min(x, y) (x < y ? x : y)
using namespace std;
typedef pair<int, int> pii;
const int N = 21 * (1e4 + 10);
const int M = 62 * (5e4 + 10);
const int inf = 0x3f3f3f3f;
struct edge
{
	int to, w, next;
}e[M];
int top, h[N], dist[N];
int n, m, k, s, t, a[N], b[N], c[N];
bool vis[N];
priority_queue<pii, vector<pii>, greater<pii> > q;

void add(int x, int y, int w)
{
	++top;
	e[top].to = y;
	e[top].w = w;
	e[top].next = h[x];
	h[x] = top;
}

void build()
{
	for (re p = 0; p < k; p ++)
	{
		for (re x = 1 + p * n; x <= n + p * n; x ++)
		{
			for (re i = h[x]; i; i = e[i].next)
			{
				int y = e[i].to;
				y += ((p + 1) * n);
				add(x, y, 0);
//				add(y, x, 0); <--- tip 1
			}
		}
		for (re i = 1; i <= m; i ++)
		{
			int delta = (p + 1) * n;
			add(a[i] + delta, b[i] + delta, c[i]);
			add(b[i] + delta, a[i] + delta, c[i]);
		}
	}
	n = n * (k + 1);
}

void dijkstra()
{
	for (re i = 1; i <= n; i ++) dist[i] = inf;
	dist[s] = 0;
	q.push({0, s});
	while (!q.empty())
	{
		int x = q.top().second; q.pop();
		if (vis[x]) continue;
		vis[x] = true;
		for (re i = h[x]; i ; i = e[i].next)
		{
			int y = e[i].to, w = e[i].w;
			if (dist[x] + w < dist[y])
			{
				dist[y] = dist[x] + w;
				q.push({dist[y], y});
			}
		}
	}
}

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0); cout.tie(0);
	
	cin >> n >> m >> k >> s >> t;
	s ++;
	t ++;
	for (re i = 1; i <= m; i ++)
	{
		cin >> a[i] >> b[i] >> c[i];
		a[i] ++;
		b[i] ++;
		add(a[i], b[i], c[i]);
		add(b[i], a[i], c[i]);
	}
	int nn = n; // <--- tip 3
	build();
	dijkstra();
//	cout << dist[t * (k + 1)]; <--- tip 2
    int ans = inf;
    for (re i = 0; i <= k; i ++)
    {
        ans = min(ans, dist[t + i * nn]);
    }
    cout << ans;
	return 0;
}