题意：给你一个有向图， 并指定起点和终点。
问要从起点走向终点， 再从终点走向起点， 最少需要走过多少不同的节点。
对于 100%的数据， 有 N<=100， M<=min(1000,N*N)。 图中可能有重边或者自环
思路：
1 const oo=700000000; 2 var head1,head2,vet1,vet2,next1,next2:array[1..10000]of longint; 3 q:array[0..100000]of record 4 x,y:longint; 5 end; 6 dis,f:array[1..100,1..100]of longint; 7 inq:array[1..100,1..100]of boolean; 8 n,m,i,j,k,cas,tot1,tot2,x,y,z:longint; 9 10 function min(x,y:longint):longint; 11 begin 12 if x<y then exit(x); 13 exit(y); 14 end; 15 16 procedure spfa; 17 var i,j,u1,u2,e1,e2,v1,v2,t,w,t1,w1,tmp:longint; 18 begin 19 for i:=1 to n do 20 for j:=1 to n do 21 begin 22 inq[i,j]:=false; 23 dis[i,j]:=oo; 24 end; 25 t:=0; w:=1; t1:=0; w1:=1; 26 q[1].x:=1; q[1].y:=1; inq[1,1]:=true; dis[1,1]:=1; 27 while t<w do 28 begin 29 inc(t); inc(t1); 30 if t1=(n*n)<<1 then t1:=0; 31 u1:=q[t1].x; u2:=q[t1].y; inq[u1,u2]:=false; 32 33 e1:=head1[u1]; 34 while e1<>0 do 35 begin 36 v1:=vet1[e1]; 37 tmp:=dis[u1,u2]; 38 if v1<>u2 then inc(tmp); 39 if tmp<dis[v1,u2] then 40 begin 41 dis[v1,u2]:=tmp; 42 if not inq[v1,u2] then 43 begin 44 inc(w); inc(w1); 45 if w1=(n*n)<<1 then w1:=0; 46 q[w1].x:=v1; q[w1].y:=u2; inq[v1,u2]:=true; 47 end; 48 end; 49 e1:=next1[e1]; 50 end; 51 52 e2:=head2[u2]; 53 while e2<>0 do 54 begin 55 v2:=vet2[e2]; 56 tmp:=dis[u1,u2]; 57 if v2<>u1 then inc(tmp); 58 if tmp<dis[u1,v2] then 59 begin 60 dis[u1,v2]:=tmp; 61 if not inq[u1,v2] then 62 begin 63 inc(w); inc(w1); 64 if w1=(n*n)<<1 then w1:=0; 65 q[w1].x:=u1; q[w1].y:=v2; inq[u1,v2]:=true; 66 end; 67 end; 68 e2:=next2[e2]; 69 end; 70 71 72 if (u1<>u2)and(dis[u1,u2]+f[u1,u2]-1<dis[u2,u1]) then 73 begin 74 dis[u2,u1]:=dis[u1,u2]+f[u1,u2]-1; 75 if not inq[u2,u1] then 76 begin 77 inc(w); inc(w1); 78 if w1=(n*n)<<1 then w1:=0; 79 q[w1].x:=u2; q[w1].y:=u1; inq[u2,u1]:=true; 80 end; 81 end; 82 end; 83 end; 84 85 procedure add1(a,b:longint); 86 begin 87 inc(tot1); 88 next1[tot1]:=head1[a]; 89 vet1[tot1]:=b; 90 head1[a]:=tot1; 91 end; 92 93 procedure add2(a,b:longint); 94 begin 95 inc(tot2); 96 next2[tot2]:=head2[a]; 97 vet2[tot2]:=b; 98 head2[a]:=tot2; 99 end; 100 101 begin 102 assign(input,'uva1057.in'); reset(input); 103 assign(output,'uva1057.out'); rewrite(output); 104 while not eof do 105 begin 106 read(n,m); 107 if n=0 then break; 108 for i:=1 to n do 109 begin 110 head1[i]:=0; 111 head2[i]:=0; 112 end; 113 tot1:=0; tot2:=0; 114 inc(cas); 115 writeln('Network ',cas); 116 for i:=1 to n do 117 for j:=1 to n do 118 if i<>j then f[i,j]:=oo; 119 for i:=1 to m do 120 begin 121 read(x,y); 122 f[x,y]:=1; 123 add1(x,y); 124 add2(y,x); 125 end; 126 for i:=1 to n do 127 for j:=1 to n do 128 for k:=1 to n do f[j,k]:=min(f[j,k],f[j,i]+f[i,k]); 129 if (f[1,2]=oo)or(f[2,1]=oo) then 130 begin 131 writeln('Impossible'); 132 writeln; 133 continue; 134 end; 135 spfa; 136 writeln('Minimum number of nodes = ',dis[2,2]); 137 writeln; 138 end; 139 close(input); 140 close(output); 141 end.