第K短路（A*）

#include <bits/stdc++.h>
using namespace std;
typedef pair<int,int> PII;
typedef pair<int,PII> PIII;
const int N=1010,M=2e5+10;
int head1[M<<1],head2[M<<1];
struct node{
    int next,to,val;
}edge[M<<1];
int uuu,S,T;
void add(int *head,int from,int to,int val){
    edge[++uuu]={ head[from],to,val };
    head[from]=uuu;
}
int dis[M<<1];
bool vis[M<<1];
void dij()
{
    priority_queue<PII,vector<PII>,greater<PII>>q;
    q.push({0,T});///距离，标号
    memset(dis,0x3f,sizeof dis);
    dis[T]=0;
    while(!q.empty()){
        auto temp=q.top();q.pop();
        if(vis[temp.second])continue;
        vis[temp.second]=true;
        for(int i=head2[temp.second];i;i=edge[i].next){
            int x=edge[i].to;
            if(dis[x]>dis[temp.second]+edge[i].val){
                dis[x]=dis[temp.second]+edge[i].val;
                q.push({dis[x],x});
            }
        }
    }
}
int K;
int astar(){
    priority_queue<PIII,vector<PIII>,greater<PIII>>q;
    q.push({dis[S],{0,S}});///估计值，真实值，标号
    int cnt=0;
    if(dis[S]==0x3f3f3f3f)return -1;
    while(!q.empty()){
        auto temp=q.top();q.pop();
        int x=temp.second.second,y=temp.second.first;
        if(x==T)cnt++;
        if(cnt==K)return y;
        for(int i=head1[x];i;i=edge[i].next){
            int xx=edge[i].to;
            q.push({dis[xx]+y+edge[i].val,{y+edge[i].val,xx}});
        }
    }
    return -1;
}
int main()
{
    ios_base::sync_with_stdio(false);
    int n,m;
    cin>>n>>m;
    for(int i=0,u,v,val;i<m;i++){
        cin>>u>>v>>val;
        add(head1,u,v,val);
        add(head2,v,u,val);
    }
    cin>>S>>T>>K;
    if(S==T)K++;
    dij();
    cout<<astar();
}