第k*短路模板（单项边）
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <queue>
#define Max 100005
#define inf 1<<28
using namespace std;
int S,T,K,n,m;
int head[Max],rehead[Max];
int num,renum;
int dis[Max];
bool visit[Max];
int ans[Max];
int qe[Max*10];
struct kdq{int v,len,next;
} edge[Max],reedge[Max];struct a_star {               //A*搜索时的优先级队列;int v;int len;bool operator<(const a_star &a)const{    //f(i)=d[i]+g[i]return len+dis[v]>a.len+dis[a.v];}
};
void insert(int u,int v,int len){//正图和逆图edge[num].v=v;edge[num].len=len;edge[num].next=head[u];head[u]=num;num++;reedge[renum].v=u;reedge[renum].len=len;reedge[renum].next=rehead[v];rehead[v]=renum;renum++;
}void init(){memset(ans,0,sizeof(ans));for(int i=0; i<=n; i++)head[i]=-1,rehead[i]=-1;num=1,renum=1;
}
void spfa(){//从T开始求出T到所有点的 dis[]int i,j;for(i=1; i<=n; i++)dis[i]=inf;dis[T]=0;visit[T]=1;int num=0,cnt=0;qe[num++]=T;while(num>cnt){int temp=qe[cnt++];visit[temp]=0;for(i=rehead[temp]; i!=-1 ; i=reedge[i].next){int tt=reedge[i].v;int ttt=reedge[i].len;if(dis[tt]>dis[temp]+ttt){dis[tt]=dis[temp]+ttt;if(!visit[tt]){qe[num++]=tt;visit[tt]=1;}}}}
}
int A_star(){if(S==T)K++;if(dis[S]==inf)return -1;a_star n1;n1.v=S;n1.len=0;priority_queue <a_star> q;q.push(n1);while(!q.empty()){a_star temp=q.top();q.pop();ans[temp.v]++;if(ans[T]==K)//当第K次取到T的时候，输出路程return temp.len;if(ans[temp.v]>K)continue;for(int i=head[temp.v]; i!=-1; i=edge[i].next){a_star n2;n2.v=edge[i].v;n2.len=edge[i].len+temp.len;q.push(n2);}}return -1;
}
int main(){int i,j,k,l;int a,b,s;scanf("%d%d",&n,&m);init();while(m--){scanf("%d%d%d",&a,&b,&s);insert(a,b,s);}scanf("%d%d%d",&S,&T,&K);spfa();printf("%d\n",A_star());return 0;
}