poj-1753

Flip Game

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 17984		Accepted: 7780

Description

Flip game is played on a rectangular 4x4 field with two-sided pieces placed on each of its 16 squares. One side of each piece is white and the other one is black and each piece is lying either it's black or white side up. Each round you flip 3 to 5 pieces, thus changing the color of their upper side from black to white and vice versa. The pieces to be flipped are chosen every round according to the following rules:

1. Choose any one of the 16 pieces.
   
2. Flip the chosen piece and also all adjacent pieces to the left, to the right, to the top, and to the bottom of the chosen piece (if there are any).

Consider the following position as an example:

bwbw
wwww
bbwb
bwwb
Here "b" denotes pieces lying their black side up and "w" denotes pieces lying their white side up. If we choose to flip the 1st piece from the 3rd row (this choice is shown at the picture), then the field will become:

bwbw
bwww
wwwb
wwwb
The goal of the game is to flip either all pieces white side up or all pieces black side up. You are to write a program that will search for the minimum number of rounds needed to achieve this goal.

Input

The input consists of 4 lines with 4 characters "w" or "b" each that denote game field position.

Output

Write to the output file a single integer number - the minimum number of rounds needed to achieve the goal of the game from the given position. If the goal is initially achieved, then write 0. If it's impossible to achieve the goal, then write the word "Impossible" (without quotes).

Sample Input

bwwb
bbwb
bwwb
bwww

Sample Output

4

Source

Northeastern Europe 2000

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通過位運算將棋盤一一對應到0..65535的數字，反轉就是xor運算，寬搜就可以了.

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Code:

program poj_1753;
const state:array[0..15]of longint=(19,39,78,140,305,626,1252,2248,4880,10016,20032,35968,12544,29184,58368,51200);
var dis,que:array[0..65535] of longint;
    top,last,start,i:longint;
    ch:char;
procedure expand(x:longint);
var i,y:longint;
begin
     if (x<0)or(x>65535)then writeln('error');
     for i:=0 to 15 do begin
         y:=x xor state[i];
         if dis[y]=maxlongint then begin
            dis[y]:=dis[x]+1;
            inc(last);
            que[last]:=y;
         end;
     end;
end;
function min(a,b:longint):longint;
begin
     if a<b then exit(a) else exit(b);
end;
begin
     start:=0;
     for i:=0 to 15 do begin
         repeat read(ch); until (ch='b')or(ch='w');
         if ch='b' then start:=start or (1shl(i));
     end;
     for i:=0 to 65535 do dis[i]:=maxlongint;
     dis[start]:=0;
     top:=0; last:=1; que[last]:=start;
     while top<last do begin
           inc(top);
           expand(que[top]);
     end;
     if min(dis[0],dis[65535])<maxlongint then
        write(min(dis[0],dis[65535]))
     else write('Impossible');
end.