poj1753

简单的BFS，加上用位操作来定义和改写状态。ac时运行了100多ms，看discussion里面有人跑的很快，应该有更优化的方法吧，没细究了。

#include <cstdio>
#include <queue>
using namespace std;

const int ROW = 4;
const int ajacent[4][2] = { { -1, 0 }, { 1, 0 }, { 0, -1 }, { 0, 1 } };

struct Status{
    int s;
    int step;
};

int getStatus(char** board)
{
    int ans = 0;
    for (int i = 0; i < ROW; i++){
        for (int j = 0; j < ROW; j++){
            ans <<= 1;
            if (board[i][j] == 'b'){
                ans++;
            }
            
        }
    }
    return ans;
}

int main()
{
    char* board[ROW];
    bool visited[65536];
    memset(visited, 0, sizeof(visited));
    queue<Status> Q;
    for (int i = 0; i < ROW; i++){
        board[i] = (char*)malloc(5 * sizeof(char));
    }
    for (int i = 0; i < ROW; i++){
        scanf("%s", board[i]);
    }
    Status tmpSt;
    tmpSt.s = getStatus(board);
    tmpSt.step = 0;
    Q.push(tmpSt);
    visited[tmpSt.s] = true;
    while (!Q.empty()){
        Status current = Q.front();
        if (current.s == 0 || current.s == 0xffff)
            break;
        for (int i = 0; i < ROW; i++){
            for (int j = 0; j < ROW; j++){
                int tmpS = current.s ^ (1 << (ROW * i + j));
                for (int k = 0; k < 4; k++){
                    int tmpI = i + ajacent[k][0],
                        tmpJ = j + ajacent[k][1];
                    if (tmpI >= 0 && tmpI < 4 && tmpJ >= 0 && tmpJ < 4){
                        tmpS ^= (1 << (ROW * tmpI + tmpJ));
                    }
                }
                if (!visited[tmpS]){
                    tmpSt.s = tmpS;
                    tmpSt.step = current.step + 1;
                    Q.push(tmpSt);
                    visited[tmpS] = true;
                }
            }
        }
        Q.pop();
    }
    if (!Q.empty()){
        printf("%d\n", Q.front().step);
    }
    else{
        printf("Impossible\n");
    }
    return 0;
}