OpenJudge/Poj 1753 Flip Game

1.链接地址：

http://bailian.openjudge.cn/practice/1753/

http://poj.org/problem?id=1753

2.题目：

总时间限制:

1000ms

内存限制:

65536kB

描述

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.

输入

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

输出

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).

样例输入

bwwb
bbwb
bwwb
bwww

样例输出

4

来源

Northeastern Europe 2000

3.思路：

4.代码：

#include "assert.h"
 #include <iostream>
 #include <queue>
#include <cstring>
#include <cstdlib>
 using namespace std;
 
 const int MAX_STATE = 65536;
 const int ALL_WHILE_STATE = 0;
 const int ALL_BLACK_STATE = 65535;
 const int WIDTH_OF_BOARD = 4;
 const int SIZE_OF_BOARD = WIDTH_OF_BOARD * WIDTH_OF_BOARD; // 4 * 4
 
 int ConvertPieceColorToInt(char color)
 {
     switch(color)
     {
     case 'b':
         return 1;
     case 'w':
         return 0;
     }
 }
 
 int FlipPiece(int state_id, int position)
 {
     state_id ^= (1 << position);
 
     // up
     if(position - 4 >= 0)
         state_id ^= (1 << (position - 4));
     // down
     if(position + 4 < SIZE_OF_BOARD)
         state_id ^= (1 << (position + 4));
     // left
     if(position % 4 != 0)
         state_id ^= (1 << (position - 1));
     // right
     if(position % 4 != 3)
         state_id ^= (1 << (position + 1));
 
     return state_id;
 }
 
 int main()
 {
     int current_state_id = 0;
     int state[MAX_STATE];
     queue<int> search_queue;
 
     memset(state, -1, sizeof(state));
 
     char color;
 
     for(int i = 0; i < SIZE_OF_BOARD; ++i)
     {
         cin >> color;
         current_state_id += ConvertPieceColorToInt(color) << i;
     }
 
     if(current_state_id == ALL_WHILE_STATE
         || current_state_id == ALL_BLACK_STATE)
     {
         cout << "0" << endl;
         return 0;
     }
 
     state[current_state_id] = 0;
     search_queue.push(current_state_id);
 
     int next_state_id;
 
     while(!search_queue.empty())
     {
         current_state_id = search_queue.front();
         search_queue.pop();
 
         for(int i = 0; i < SIZE_OF_BOARD; ++i)
         {
             next_state_id = FlipPiece(current_state_id, i);
             if(next_state_id == ALL_WHILE_STATE
                 || next_state_id == ALL_BLACK_STATE)
             {
                 cout << state[current_state_id] + 1 << endl;
                 return 0;
             }
             assert(next_state_id < MAX_STATE);
             if(state[next_state_id] == -1 /* not visited */)
             {
                 state[next_state_id] = state[current_state_id] + 1;
                 search_queue.push(next_state_id);
             }
         }
     }
 
     cout << "Impossible" << endl;
     return 0;
}