[Usaco2007 Jan]Telephone Lines架设电话线

 1 #include<cstdio>
 2 #include<cstring>
 3 #define inf 0x7fffffff
 4 struct poi{int to,nxt,V;}e[20005];
 5 int cnt,head[1005];
 6 inline void add(int u,int v,int V){
 7     e[++cnt]=(poi){v,head[u],V}; 
 8     head[u]=cnt;
 9 }
10 int dis[1005],q[10005];
11 bool vis[1005];
12 int n,m,k,ans;
13 inline int max(int a,int b){return a>b?a:b;}
14 inline int min(int a,int b){return a<b?a:b;}
15 void spfa(int u,int mx) {
16     for(int i=1;i<=n;i++) dis[i]=inf;
17     dis[1]=0; q[0]=u; vis[u]=1;
18     int t=0,w=1;
19     while(t!=w) {
20         int now=q[t++];
21         for(int i=head[now];i;i=e[i].nxt) {
22             int to=e[i].to;
23             if((dis[now]+((e[i].V>mx)?1:0))<dis[to]) {
24                 dis[to]=dis[now]+((e[i].V>mx)?1:0);
25                 if(!vis[to]) {
26                     vis[to]=1; q[w++]=to;
27                 }
28             }
29         }
30         vis[now]=0;
31     }
32 }
33 inline bool check(int mx) {
34     spfa(1,mx);
35     return dis[n]<=k;
36 }
37 int main() {
38     scanf("%d%d%d",&n,&m,&k);
39     int l=0,r=0;
40     for(int i=1;i<=m;i++) {
41         int u,v,V;
42         scanf("%d%d%d",&u,&v,&V);
43         if(u==v) continue;
44         add(u,v,V); add(v,u,V);
45         r=max(r,V);
46     }
47     ans=-1;
48     while(l<=r) {
49         int mid=(l+r) >> 1;
50         if(check(mid)){ans=mid;r=mid-1;}
51         else l=mid+1;
52     }
53     printf("%d\n",ans);
54 }