题意：

给定一个有 n 个点，m 条边的有向带权图，求 s 到 t 的第 k 短路。(n, k <= 1e3, m <= 1e5)

链接：

https://vjudge.net/problem/POJ-2449

解题思路：

k 短路练手，注意特判 s == t 。

参考代码：

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<vector>
#include<queue>

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
#define pb push_back
#define sz(a) ((int)a.size())
#define mem(a, b) memset(a, b, sizeof a)
#define lson (rt << 1)
#define rson (rt << 1 | 1)
#define gmid (l + r >> 1)
const int maxn = 1e3 + 5;
const int maxm = 1e6 + 5;
const int mod = 1e9 + 7;
const int inf = 0x3f3f3f3f;

struct Node{
	int ls, rs, dis, val, to;
} tr[maxm];
vector<pii> G1[maxn], G2[maxn];
priority_queue<pii, vector<pii>, greater<pii> > q;
int dis[maxn], fa[maxn], rt[maxn];
int n, m, sp, tp, k, tot;

int merge(int x, int y, int f){

	if(!x || !y) return x + y;
	if(tr[x].val > tr[y].val) swap(x, y);
	if(f) tr[++tot] = tr[x], x = tot;
	tr[x].rs = merge(tr[x].rs, y, f);
	if(tr[tr[x].ls].dis < tr[tr[x].rs].dis) swap(tr[x].ls, tr[x].rs);
	tr[x].dis = tr[tr[x].rs].dis + 1;
	return x;
}

void init(){

	for(int i = 1; i <= n; ++i) G1[i].clear(), G2[i].clear();
	for(int i = 1; i <= n; ++i) fa[i] = rt[i] = 0;
	while(!q.empty()) q.pop();
	tot = 0;
}

void dij(int s, vector<pii> G[]){

	for(int i = 1; i <= n; ++i) dis[i] = inf;
	dis[s] = 0, q.push({dis[s], s});
	while(!q.empty()){

		int u = q.top().second, d = q.top().first; q.pop();
		if(d != dis[u]) continue;
		for(int i = 0; i < sz(G[u]); ++i){

			int v = G[u][i].second, w = G[u][i].first;
			if(dis[v] > dis[u] + w){

				dis[v] = dis[u] + w;
				q.push({dis[v], v});
			}
		}
	}
}

void build(vector<pii> G[]){

	for(int u = 1; u <= n; ++u){

		if(dis[u] == inf) continue;
		for(int i = 0; i < sz(G[u]); ++i){

			int v = G[u][i].second, w = G[u][i].first;
			if(dis[u] == dis[v] + w && !fa[u]) fa[u] = v;
			else if(dis[v] != inf){

				tr[++tot] = {0, 0, 0, dis[v] + w - dis[u], v};
				rt[u] = merge(rt[u], tot, 0);
			}
		}
		q.push({dis[u], u});
	}
	while(!q.empty()){

		int u = q.top().second; q.pop();
		if(fa[u]) rt[u] = merge(rt[u], rt[fa[u]], 1);
	}
}

int solve(){

	dij(tp, G2); build(G1);
	if(sp == tp) ++k;
	if(!rt[sp]) return -1;
	if(k == 1) return dis[sp];
	int res = k - 1;
	q.push({tr[rt[sp]].val, rt[sp]});
	while(!q.empty()){

		int u = q.top().second, d = q.top().first; q.pop();
		if(--res == 0) return dis[sp] + d;
		if(int v = tr[u].ls) q.push({d - tr[u].val + tr[v].val, v});
		if(int v = tr[u].rs) q.push({d - tr[u].val + tr[v].val, v});
		if(int v = rt[tr[u].to]) q.push({d + tr[v].val, v});
	}
	return -1;
}

int main(){

//    ios::sync_with_stdio(0); cin.tie(0);
	while(scanf("%d%d", &n, &m) != EOF){

		init();
		for(int i = 1; i <= m; ++i){

			int u, v, w; scanf("%d%d%d", &u, &v, &w);
			G1[u].pb({w, v}), G2[v].pb({w, u});
		}
		scanf("%d%d%d", &sp, &tp, &k);
		int ret = solve();
		printf("%d\n", ret);
	}
    return 0;
}