二分最长的电话线长度。把所有大于这个长度的边权设成1，小于等于的设成零，然后跑SPFA看dis[n]是否>k。若>k则l=mid+1

　　否则r=mid-1

　　放代码

#include
     
      #include
      
       #include
       
        #include
        
         inline long long read(){    long long num=0,f=1;    char ch=getchar();    while(!isdigit(ch)){        if(ch=='-')    f=-1;        ch=getchar();    }    while(isdigit(ch)){        num=num*10+ch-'0';        ch=getchar();    }    return num*f;}struct Edge{    int next,to,dis;}edge[1000010];int head[100100],num;inline void add(int from,int to,int dis){    edge[++num]=(Edge){head[from],to,dis};    head[from]=num;}int val[100010];int f[1000001],h,t=1;bool vis[100010];int dis[100010];int ans;int main(){    int n=read(),m=read(),k=read();    for(int i=1;i<=m;++i){        int from=read(),to=read(),dst=read();        add(from,to,dst);        add(to,from,dst);        val[i]=dst;    }    std::sort(val+1,val+m+1);    int l=1,r=m;        while(l<=r){        int mid=(l+r)>>1;        int cost=val[mid];        memset(vis,0,sizeof(vis));        memset(dis,127/3,sizeof(dis));        f[1]=1;h=0;t=1;dis[1]=0;        while(h++
         
          dis[from]+c){                    dis[to]=dis[from]+c;                    if(!vis[to]){                        vis[to]=1;                        f[++t]=to;                    }                }            }        }        if(dis[n]>k)    l=mid+1;        else{            ans=cost;            r=mid-1;        }    }    printf("%d",ans==0?-1:ans);    return 0;}