poj3041 Asteroids（二分图最小顶点覆盖、二分图匹配）

 1 program rrr(input,output);
 2 const
 3   inf=123456789;
 4 type
 5   etype=record
 6      t,c,rev,next:longint;
 7   end;
 8 var
 9   e:array[0..20020]of etype;
10   a,cur,d:array[-505..505]of longint;
11   q:array[0..1010]of longint;
12   n,m,i,x,y,h,t,cnt,ans:longint;
13 procedure ins(x,y,c:longint);
14 begin
15    inc(cnt);e[cnt].t:=y;e[cnt].c:=c;e[cnt].next:=a[x];a[x]:=cnt;
16 end;
17 procedure add(x,y:longint);
18 begin
19    ins(x,y,1);ins(y,x,0);
20    e[cnt].rev:=cnt-1;e[cnt-1].rev:=cnt;
21 end;
22 function min(a,b:longint):longint;
23 begin
24    if a<b then exit(a) else exit(b);
25 end;
26 procedure bfs;
27 begin
28    for i:=-n to n+1 do d[i]:=-1;
29    h:=0;t:=1;q[1]:=0;d[0]:=0;
30    while h<t do
31       begin
32          inc(h);
33          i:=a[q[h]];
34          while i<>inf do
35             begin
36                if (d[e[i].t]=-1) and (e[i].c>0) then
37                   begin
38                      d[e[i].t]:=d[q[h]]+1;
39                      inc(t);q[t]:=e[i].t;
40                   end;
41                i:=e[i].next;
42             end;
43       end;
44 end;
45 function dfs(k,f:longint):longint;
46 var
47   ans,r,i:longint;
48 begin
49    if (k=n+1) or (f=0) then exit(f);
50    ans:=0;i:=cur[k];
51    while i<>inf do
52       begin
53          if (e[i].c>0) and (d[e[i].t]=d[k]+1) then
54             begin
55                r:=dfs(e[i].t,min(f,e[i].c));
56                dec(e[i].c,r);inc(e[e[i].rev].c,r);
57                ans:=ans+r;f:=f-r;
58                if f=0 then break;
59             end;
60          i:=e[i].next;
61          cur[k]:=i;
62       end;
63    if f>0 then d[k]:=-1;
64    exit(ans);
65 end;
66 begin
67    assign(input,‘r.in‘);assign(output,‘r.out‘);reset(input);rewrite(output);
68    readln(n,m);
69    cnt:=0;for i:=-n to n+1 do a[i]:=inf;
70    for i:=1 to n do begin add(0,-i);add(i,n+1); end;
71    for i:=1 to m do begin readln(x,y);add(-x,y); end;
72    ans:=0;
73    while true do
74       begin
75          bfs;
76          if d[n+1]=-1 then break;
77          for i:=-n to n+1 do cur[i]:=a[i];
78          ans:=ans+dfs(0,inf);
79       end;
80    write(ans);
81    close(input);close(output);
82 end.