例题：Flip Game

#include <iostream>
#include <algorithm>
#include <map>
#include <cstdio>
using namespace std;
#define ll long long
const int INF = 0x3f3f3f3f;     ///1 061 109 567
const int negative_infinite = 0xcfcfcfcf;   ///-808 464 433
const ll mod = 1e9 + 7;
const double eps = 1e-8;
const int MAXN = 1e6 + 117;
const int MAXM = 1e6 + 117;

int ans;
int a[7][7];
char s[7][7];

bool check() {
    for(int i = 1; i <= 4; i++)
        for(int j = 1; j <= 4; j++)
            if(a[i][j] != a[1][1])
                return false;
    return true;
}
void change(int i, int j) {
    a[i][j] ^= 1;        //做异或处理
    a[i + 1][j] ^= 1;
    a[i - 1][j] ^= 1;
    a[i][j - 1] ^= 1;
    a[i][j + 1] ^= 1;
}
void dfs(int x, int cnt) {
    if(x == 16) {          //出口
        if(check())
            ans=min(ans,cnt);
        return;
    }
    dfs(x+1,cnt);          //从末尾开始
    change(x/4+1,x%4+1);
    dfs(x+1,cnt+1);
    change(x/4+1,x%4+1);   //注意还原！当前状态不能影响之前的状态
                           //一维数组映射为二维数组
}                          //x/m+1 为行数  x%m+1为列数
int main() {
    for(int i = 0; i < 4; i++)
        scanf("%s", s[i]);
    for(int i = 1; i <= 4; i++)
        for(int j = 1; j <= 4; j++)
            if(s[i - 1][j - 1] == 'b')
                a[i][j] = 1;
    ans = INF;
    dfs(0, 0);
    if(ans == INF)
        puts("Impossible");
    else
        printf("%d\n", ans);
    return 0;
}