[SHTSC 2007] 善意的投票

我就是来复习Dinic算法的，仅10天不写，我已经退化成写一遍+调试需要接近一个小时了，当然其中不乏在网上乱逛的时间…

赞成从S源点连一条单向边，反对向T汇点连一条单向边，朋友关系连双向边。

但是总感觉自己看到题目不能一下想到这是网络流，感觉这些题都是给一个图，求最优之类。

program vote;
type ptype=^node;
     node=record
          v,w,flow:longint;
          op,next:ptype;
          end;
const maxn=310;
var head:array[1..maxn] of ptype;
    visit:array[1..maxn] of boolean;
    q,d:array[0..maxn] of longint;
    n,m,i,t,x,y,st,ed:longint;

function min(a,b:longint):longint;
begin
  if a<b then exit(a) else exit(b);
end;

procedure insert(u,v,r,s:longint);
var p1,p2,q:ptype;
begin
  new(p1);
  p1^.v:=v;p1^.w:=r;p1^.flow:=0;p1^.next:=nil;
  q:=head[u];
  if q=nil then head[u]:=p1 else
    begin
      while q^.next<>nil do q:=q^.next;
      q^.next:=p1;
    end;
  new(p2);
  p2^.v:=u;p2^.w:=s;p2^.flow:=0;p2^.next:=nil;
  q:=head[v];
  if q=nil then head[v]:=p2 else
    begin
      while q^.next<>nil do q:=q^.next;
      q^.next:=p2;
    end;
  p1^.op:=p2;p2^.op:=p1;
end;

function bfs:boolean;
var star,rear,x:longint;
    y:ptype;
begin
  fillchar(visit,sizeof(visit),false);
  fillchar(q,sizeof(q),0);fillchar(d,sizeof(d),0);
  star:=1;rear:=1;q[star]:=st;visit[st]:=true;d[st]:=1;
  while star<=rear do
    begin
      x:=q[star];y:=head[x];
      while y<>nil do
        begin
          if (not visit[y^.v]) and (y^.w>y^.flow) then
            begin
              inc(rear);
              q[rear]:=y^.v;
              visit[y^.v]:=true;
              d[y^.v]:=d[x]+1;
            end;
          y:=y^.next;
        end;
      inc(star);
    end;
  bfs:=visit[ed];
end;

function addflow(p,maxflow:longint):longint;
var y:ptype;
    o:longint;
begin
  if (p=ed) or (maxflow=0) then exit(maxflow);
  //fillchar(visit,sizeof(visit),false);
  visit[p]:=true;
  addflow:=0;
  y:=head[p];
  while y<>nil do
    begin
      if (d[y^.v]=d[p]+1) and (y^.w>y^.flow) then
        begin
          o:=addflow(y^.v,min(maxflow,y^.w-y^.flow));
          if o>0 then
            begin
              inc(addflow,o);
              dec(maxflow,o);
              inc(y^.flow,o);
              dec(y^.op^.flow,o);
              if maxflow=0 then exit;
            end;
        end;
      y:=y^.next
    end;
end;

function network:longint;
begin
  network:=0;
  while bfs do
    inc(network,addflow(st,maxlongint));
end;

begin
  //assign(input,'vote.in');reset(input);
  //assign(output,'vote.out');rewrite(output);
  readln(n,m);
  st:=0;ed:=n+1;
  for i:=1 to n do
    begin
      read(t);
      if t=1 then insert(st,i,1,0) else insert(i,ed,1,0);
    end;
  for i:=1 to m do
    begin
      readln(x,y);
      insert(x,y,1,1);
    end;
  writeln(network);
  //close(input);close(output);
end.

P.S. 07年该是网络流刚开始风靡那会儿吧，怪不得时限5s，数据也很小…