[USACO08JAN]电话线Telephone Lines [二分 01BFS]

[USACO08JAN]电话线Telephone Lines

二分+01BFS

#include<bits/stdc++.h>
using namespace std;
#define Max(x,y) (x)>(y)?(x):(y)
#define Min(x,y) (x)>(y)?(y):(x)
#define ll long long
#define rg register
const int N=1000+5,M=10000+5,inf=0x3f3f3f3f,P=99999997;
int n,m,k,mx;
template <class t>void rd(t &x){
    x=0;int w=0;char ch=0;
    while(!isdigit(ch)) w|=ch=='-',ch=getchar();
    while(isdigit(ch)) x=(x<<1)+(x<<3)+(ch^48),ch=getchar();
    x=w?-x:x;
}

int head[N],tot=0;
struct edge{int v,w,nxt;}e[M<<1];
void add(int u,int v,int w){
    e[++tot]=(edge){v,w,head[u]},head[u]=tot;
}

int dis[N];
bool vis[N];
int check(int x){
    memset(dis,0,sizeof(dis));
    memset(vis,0,sizeof(vis));
    deque<int>q;
    q.push_back(1),dis[1]=0,vis[1]=1;
    int flag=0;
    while(!q.empty()){
        int u=q.front();
        q.pop_front();
        for(int i=head[u],v,w;i;i=e[i].nxt){
            v=e[i].v,w=e[i].w;
            if(!vis[v]||dis[v]>=dis[u]+1){
                if(w<=x) vis[v]=1,q.push_front(v),dis[v]=dis[u];
                else vis[v]=1,q.push_back(v),dis[v]=dis[u]+1;
                if(v==n) flag=1;
            }
        }
    }
    if(!flag) return 2;
    if(dis[n]>k) return 0;
    else return 1;
}

int main(){
//    freopen("in2.txt","r",stdin);
    rd(n),rd(m),rd(k);
    for(int i=1,u,v,w;i<=m;++i)
    rd(u),rd(v),rd(w),add(u,v,w),add(v,u,w),mx=Max(mx,w);
    int l=1,r=mx;
    if(check(inf)==2) {puts("-1");return 0;}
    while(l<r){
        int mid=l+r>>1;
        if(check(mid)) r=mid;
        else l=mid+1;
    }
    if(check(0)) puts("0");
    else if(l!=1) printf("%d",l);
    else puts("-1");
    return 0;
}