noi05-聪聪和可可

这题还是比较容易的，不过第一次时写挂了。。。主要思路就是类似spfa的方法预处理得到next数组，然后就是记忆化搜索，我没用链式邻接表，但既然数组模拟没Mle，那就不该了吧。。
贴代码（这个是我最近写的最短的代码了）：

const     maxn=1010;var   g,dist,next:array[1..maxn,0..maxn] of longint;   vis:array[1..maxn] of boolean;   q:array[0..maxn] of longint;   f:array[1..maxn,1..maxn] of double;   N,E,C,M:longint;procedure init;var   i,a,b:longint;begin     readln(N,E);     readln(C,M);     for i:=1 to N do g[i,0]:=0;     for i:=1 to E do begin         readln(a,b);         inc(g[a,0]); g[a,g[a,0]]:=b;         inc(g[b,0]); g[b,g[b,0]]:=a;     end;     fillchar(f,sizeof(f),0);end;procedure path;var   top,last,i,j,now:longint;begin     for i:=1 to N do     for j:=1 to N do dist[i,j]:=maxlongint;     for i:=1 to N do begin         dist[i,i]:=0;         next[i,i]:=i;         fillchar(vis,sizeof(vis),false);         vis[i]:=true;         top:=0; last:=1; q[last]:=i;         while top<last do begin               inc(top);               now:=q[top];               vis[now]:=false;               for j:=1 to g[now,0] do                   if (dist[i,now]+1<dist[i,g[now,j]])or((dist[i,now]+1=dist[i,g[now,j]])and(now<next[g[now,j],i])) then begin                      dist[i,g[now,j]]:=dist[i,now]+1;                      next[g[now,j],i]:=now;                      if not vis[g[now,j]] then begin                         inc(last);                         q[last]:=g[now,j];                         vis[g[now,j]]:=true;                      end;                   end;         end;     end;end;function getf(p,q:longint):double;var   sum:double;   i:longint;begin     if p=q then exit(0);     p:=next[p,q]; if p=q then exit(1);     p:=next[p,q]; if p=q then exit(1);     if f[p,q]>1e-9 then exit(f[p,q]);     sum:=getf(p,q);     for i:=1 to g[q,0] do         sum:=sum+getf(p,g[q,i]);     sum:=sum/(g[q,0]+1)+1;     getf:=sum;     f[p,q]:=sum;end;procedure main;begin     assign(input,'cchkk.in');     assign(output,'cchkk.out');     reset(input);     rewrite(output);     init;     path;     write(getf(C,M):0:3);     close(input);     close(output);end;begin     main;end.