POJ 1860（判定正圈）

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Bellman_ford

Program P1860;var   n,m,i,j,s:longint;   v:double;   flag:boolean;   d:array[1..100] of double;   x,y:array[1..100] of longint;   map:array[1..100,1..4] of double;procedure relax(i:longint);begin   if (d[y[i]]<(d[x[i]]-map[i,2])*map[i,1]) then   begin      d[y[i]]:=(d[x[i]]-map[i,2])*map[i,1];      flag:=false;   end;   if (d[x[i]]<(d[y[i]]-map[i,4])*map[i,3]) then   begin      d[x[i]]:=(d[y[i]]-map[i,4])*map[i,3];      flag:=false;   end;end;procedure bell_ford;var   i,j:longint;begin   for i:=1 to n do   begin      flag:=true;      for j:=1 to m do relax(j);      if flag then break;   end;   if flag then writeln('NO')   else writeln('YES');end;begin   while not seekeof do   begin      readln(n,m,s,v);      fillchar(d,sizeof(d),0);      d[s]:=v;      for i:=1 to m do      begin         readln(x[i],y[i],map[i,1],map[i,2],map[i,3],map[i,4]);      end;      bell_ford;   end;end.