[Usaco2007 Jan]Telephone Lines架设电话线

http://www.lydsy.com/JudgeOnline/problem.php?id=1614

二分，练手。注意处理无解和答案为0.

/**************************************************************
    Problem: 1614
    User: lijianlin1995
    Language: Pascal
    Result: Accepted
    Time:68 ms
    Memory:656 kb
****************************************************************/
 
program Telephone;
uses math;
const Filename='Telephone';
      maxn=1010;
      maxp=10010;
var v,w,next  :array[0..maxp*2] of longint;
    head,dis,q:array[0..maxn]   of longint;
    vis       :array[0..maxn]   of boolean;
    ans,x,y,z,i,n,p,k,l,r,tot,mid:longint;
procedure build(x,y,z:longint);
          begin
          inc(tot);
          v[tot]:=y;w[tot]:=z;
          next[tot]:=head[x];
          head[x]:=tot;
          end;
procedure SPFA(x:longint);
          var h,t,i,cur,cost:longint;
          begin
          fillchar (vis,sizeof(vis),        false);
          filldword(dis,sizeof(dis)>>2,maxlongint);
          h:=0;t:=1;Q[1]:=1;
          dis[1]:=0;vis[1]:=true;
          while h<>t do begin
            inc(h);if h>maxn then h:=1;
            cur:=Q[h];i:=head[cur];
            while i<>0 do begin
              cost:=longint(w[i]>x);
              if(dis[v[i]]>dis[cur]+cost)
                then begin
                     dis[v[i]]:=dis[cur]+cost;
                     if not vis[v[i]]
                       then begin
                            inc(t);if t>maxn then t:=1;
                            Q[t]:=v[i];vis[v[i]]:=true;
                            end;
                     end;
              i:=next[i];end;
            vis[cur]:=false;end;
          end;
begin
readln(N,P,K);
for i:=1 to P do
  begin
  readln(x,y,z);
  build(x,y,z);
  build(y,x,z);
  r:=max(z,r);
  end;
SPFA(r);l:=-1;
if dis[n]>k
  then ans:=-1
  else begin
       while l<r-1 do begin
         mid:=(l+r)>>1;SPFA(mid);
         if dis[n]<=k
           then r:=mid
           else l:=mid;
           end;
       ans:=r;
       end;
writeln(ans);
end.