POJ 3169(带起点的拆分约束）

这次的拆分约束有明显的起点。

拆分等式 Xb-Xa<=C

     Xa-Xb<=-C

Program P3169;
const
   maxml=10000;
   maxn=1000;
var
   n,ml,md,i,j:longint;
   a,b:array[1..maxml,1..3] of longint;
   d:array[0..maxn] of longint;
   flag:boolean;
procedure relax(v,u,w:longint);
begin
   if d[v]>d[u]+w then
   begin
      d[v]:=d[u]+w;
      flag:=true;
   end;
end;
procedure bellman_ford;
var
   i,j,k:longint;
begin
   fillchar(d,sizeof(d),127);
   d[1]:=0;
   for k:=1 to ml+md do
   begin
      flag:=false;
      for i:=1 to ml do
      begin
         relax(a[i,2],a[i,1],a[i,3]);
      end;
      for i:=1 to md do
      begin
         relax(b[i,1],b[i,2],-b[i,3]);
      end;
      if not(flag) then break;
   end;
   if flag then writeln('-1')
   else if d[n]=2139062143 then writeln('-2')
   else writeln(d[n]);



end;
begin
   read(n,ml,md);
   for i:=1 to ml do
   begin
      read(a[i,1],a[i,2],a[i,3]);
   end;
   for i:=1 to md do
   begin
      read(b[i,1],b[i,2],b[i,3]);
   end;
   bellman_ford;
end.