Ural 1060 Flip Game

Ural 1060 Flip Game

C++

Accepted

0.031

197 KB

1060. Flip Game

Time Limit: 2.0 second
Memory Limit: 16 MB

Flip game is played on a rectangular 4×4 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 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	output
bwbw wwww bbwb bwwb	Impossible

Problem Source: 2000-2001 ACM Northeastern European Regional Programming Contest

wa了几次
原因：1是cnt只增没减
             2是search的推出条件写成if(m==16){ ……}，这种情况下对于只改变最后一个即可得情况没有判定。改成m>16

#include < iostream >
using   namespace  std;
bool  status[ 6 ][ 6 ] = { 0 };
const   int  MAX = 0x7fffffff ;
int  cnt = 0 ,cntMin = MAX;        

bool  check()
{
     bool  f = status[ 1 ][ 1 ];
     int  i,j;
     for (i = 1 ; i <= 4 ; i ++ )
     for (j = 1 ; j <= 4 ; j ++ )
        if (f != status[i][j]) return   false ;
    
     return   true ;
}

void  turn( int  i,  int  j)
{
     status[i][j] =! status[i][j];
     status[i - 1 ][j] =! status[i - 1 ][j];
     status[i + 1 ][j] =! status[i + 1 ][j];
     status[i][j - 1 ] =! status[i][j - 1 ];
     status[i][j + 1 ] =! status[i][j + 1 ];
}

void  input()
{
      char  ch;
      for ( int  i = 1 ; i <= 4 ; i ++ )
      for ( int  j = 1 ; j <= 4 ; j ++ )
     {
             cin >> ch;
             status[i][j] = (ch == ' b '   ?   0  :  1  );
     }
} 

void  search( int  m)
{
         if (m > 16 ){
                  if ( check()  &&  (cnt < cntMin) )cntMin = cnt;     
                  return  ;
        }
         int  i = (m + 3 ) / 4 ; 
         int  j = m - 4 * (i - 1 );
        
        turn(i,j); cnt ++ ;
        search(m + 1 );
        
        turn(i,j);cnt -- ;
        search(m + 1 );   
}

int  main()
{
    input();   
    search( 1 );
     if (cntMin == MAX)cout << " Impossible " << endl;
     else  cout << cntMin << endl;
    system( " pause " );
     return   0 ;
}