var  e:array[1..maxe]of record a,b,w:longint;end;  { 距源点s距离 }  dis:array[1..maxn]of longint;  { 前驱 }  pre:array[1..maxn]of longint;  m,n,s:longint;procedure relax(u,v,w:longint);  begin    if dis[u]+w<dis[v] then      begin        dis[v]:=dis[u]+w;        pre[v]:=u;      end;  end;function bellman_ford:boolean;  var i,j:longint;  begin    { 每条边松弛 }    for i:=1 to n-1 do      for j:=1 to m do        with e[j] do          relax(a,b,w);    { 如果还能松弛 }    for i:=1 to m do      with e[i] do        if dis[a]+w<dis[b] then          exit(false);    exit(true)  end;