很早的時候,就聽說了dancing links,但是一直覺得多麼高深,多麼神奇,這幾天再回來看的時候,才發現這東西對搜索的優化很無語。
dancing links發現,在搜索過程中(指精確覆蓋),很多行和列被刪掉了,可是以後枚舉的時候還是會枚舉到它,於是,就用鏈表把刪掉的弄走,然後每次就可以少枚舉一些東西,剪掉了一些常數,而這個常數的減少對於n!的枚舉來說,是非常寶貴,但是,爲此,就要用醜醜的雙向鏈表來輔助(!.!寫的暈死了),可能多寫一點之後,可以感受到他的mei吧。
ps:代碼參考了hyc的,但是縮行實在比不上他,合併了兩個過程後,還是比他長些,無語了
program poj3740;
var
r,l,u,d,lin,col,size:array[0..6000]of longint;
maxn,bj,st,i,j,m,n,x:longint;
flag:boolean;
procedure inf;
begin
assign(input,'3740.in');
assign(output,'3740.out');
reset(input);rewrite(output);
end;
procedure ouf;
begin
close(input);close(output);
end;
procedure origin;
begin
fillchar(l,sizeof(l),0); fillchar(r,sizeof(r),0);
fillchar(u,sizeof(u),0); fillchar(d,sizeof(d),0);
fillchar(size,sizeof(size),0);
fillchar(lin,sizeof(lin),0);
fillchar(col,sizeof(col),0);
for i:=1 to m do
begin
l[i]:=i-1; r[i]:=i+1; u[i]:=i; d[i]:=i;
end;
l[m]:=m-1; r[m]:=0; l[0]:=m; r[0]:=1;
for i:=1 to n do
begin
l[m+i]:=m+i; r[m+i]:=m+i;
end;
st:=n+m;
end;
procedure link(x,y,xx,yy:longint);
begin
inc(st);
lin[st]:=x; col[st]:=y;
l[st]:=l[x]; r[st]:=x; l[r[st]]:=st; r[l[st]]:=st;
u[st]:=u[y]; d[st]:=y; d[u[st]]:=st; u[d[st]]:=st;
inc(size[y]);
end;
procedure d_r(x,bj:longint);
var stop,now:longint;
begin
if bj=1 then begin r[l[x]]:=r[x]; l[r[x]]:=l[x]; end;
if bj=2 then begin l[r[x]]:=x; r[l[x]]:=x; end;
stop:=x;
while d[x]<>stop do
begin
x:=d[x];
now:=x;
while r[now]<>x do
begin
now:=r[now];
if now<>lin[x] then
begin
if bj=1 then begin u[d[now]]:=u[now]; d[u[now]]:=d[now]; end;
if bj=2 then begin u[d[now]]:=now; d[u[now]]:=now; end;
end;
end;
end;
end;
procedure prepare;
begin
origin;
for i:=1 to n do
begin
for j:=1 to m do
begin
read(x);
if x=1 then link(i+m,j,i,j);
end;
readln;
end;
end;
procedure dfs(k:longint);
var
x,now,mov:longint;
begin
if r[0]=0 then begin flag:=true; exit; end;
maxn:=-maxlongint; bj:=0;
while r[bj]<>0 do
begin
bj:=r[bj];
if size[bj]>maxn then
begin
maxn:=size[bj];
x:=bj;
end;
end;
now:=x; d_r(x,1);
while d[now]<> x do
begin
now:=d[now]; mov:= now;
while r[mov]<>now do
begin
mov:=r[mov];
if mov<>lin[now] then
d_r(col[mov],1);
end;
dfs(k+1);
if flag then exit;
mov:=now;
while r[mov]<>now do
begin
mov:=r[mov];
if mov<>lin[now] then
d_r(col[mov],2);
end;
end;
d_r(x,2);
end;
procedure main;
begin
read(n,m);
prepare;
flag:=false;
dfs(0);
if flag then writeln('Yes, I found it')
else writeln('It is impossible');
end;
begin
inf;
while not eof do main;
ouf;
end.