比较简单的数学期望，先预处理出当聪聪在i，可可在j时聪聪往哪个点走
然后做dp即可，我用了记忆化搜索实现
1 type node=record 2 po,next:longint; 3 end; 4 5 var d,pos:array[0..1010,0..1010] of longint; 6 f:array[0..1010,0..1010] of double; 7 v:array[0..1010] of boolean; 8 e:array[0..2010] of node; 9 c,p,q:array[0..2010] of longint; 10 len,s,t,i,j,n,m,x,y:longint; 11 12 procedure add(x,y:longint); 13 begin 14 inc(len); 15 e[len].po:=y; 16 e[len].next:=p[x]; 17 p[x]:=len; 18 end; 19 20 procedure bfs(s:longint); 21 var f,r,i,x,y:longint; 22 begin 23 d[s,s]:=0; 24 fillchar(v,sizeof(v),false); 25 v[s]:=true; 26 f:=1; 27 r:=0; 28 i:=p[s]; 29 while i<>0 do 30 begin 31 y:=e[i].po; 32 v[y]:=true; 33 inc(r); 34 q[r]:=y; 35 d[s,y]:=1; 36 pos[s,y]:=y; 37 i:=e[i].next; 38 end; 39 while f<=r do 40 begin 41 x:=q[f]; 42 i:=p[x]; 43 while i<>0 do 44 begin 45 y:=e[i].po; 46 if not v[y] then 47 begin 48 d[s,y]:=d[s,x]+1; 49 pos[s,y]:=pos[s,x]; 50 v[y]:=true; 51 inc(r); 52 q[r]:=y; 53 end 54 else if (d[s,y]=d[s,x]+1) and (pos[s,x]<pos[s,y]) then 55 pos[s,y]:=pos[s,x]; 56 57 i:=e[i].next; 58 end; 59 inc(f); 60 end; 61 end; 62 63 function dfs(x,y:longint):double; 64 var i,w:longint; 65 begin 66 if x=y then exit(0); 67 if (pos[x,y]=y) or (pos[pos[x,y],y]=y) then exit(1); 68 if f[x,y]<>-1 then exit(f[x,y]); 69 i:=p[y]; 70 f[x,y]:=dfs(pos[pos[x,y],y],y); 71 while i<>0 do 72 begin 73 w:=e[i].po; 74 f[x,y]:=f[x,y]+dfs(pos[pos[x,y],y],w); 75 i:=e[i].next; 76 end; 77 f[x,y]:=f[x,y]/(c[y]+1)+1; 78 exit(f[x,y]); 79 end; 80 81 begin 82 readln(n,m); 83 readln(s,t); 84 for i:=1 to m do 85 begin 86 readln(x,y); 87 add(x,y); 88 add(y,x); 89 inc(c[x]); 90 inc(c[y]); 91 end; 92 for i:=1 to n do 93 begin 94 for j:=1 to n do 95 f[i,j]:=-1; 96 pos[i,i]:=i; 97 f[i,i]:=0; 98 bfs(i); 99 end; 100 writeln(dfs(s,t):0:3); 101 end.
