枚举所有情况，每个位置可能变也可能不变，复杂度O(2^16)。
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1 /* 2 Author:Zhaofa Fang 3 Lang:C++ 4 */ 5 #include <cstdio> 6 #include <cstdlib> 7 #include <iostream> 8 #include <cmath> 9 #include <cstring> 10 #include <algorithm> 11 #include <string> 12 #include <utility> 13 #include <vector> 14 #include <queue> 15 #include <stack> 16 #include <map> 17 #include <set> 18 using namespace std; 19 typedef long long ll; 20 #define pii pair<int,int> 21 #define pb push_back 22 #define mp make_pair 23 #define fi first 24 #define se second 25 #define lowbit(x) (x&(-x)) 26 #define INF 2147483647; 27 28 int tot,ans; 29 int a[17]; 30 char str[17][17]; 31 void change(int x) 32 { 33 a[x]^=1; 34 if(x-4>0) 35 { 36 a[x-4]^=1; 37 } 38 if(x-1>0 && (x-1)%4) 39 { 40 a[x-1]^=1; 41 } 42 if(x+1<17 && x%4) 43 { 44 a[x+1]^=1; 45 } 46 if(x+4<17) 47 { 48 a[x+4]^=1; 49 } 50 } 51 bool check() 52 { 53 int cnt=0; 54 for(int i=1;i<=16;i++)cnt+=a[i]; 55 if(cnt % 16 == 0)return true; 56 else return false; 57 } 58 int tmp[17]; 59 void dfs(int cur) 60 { 61 if(cur > 16) 62 { 63 if(check()) 64 { 65 if(tot < ans)ans = tot; 66 } 67 return; 68 } 69 dfs(cur+1); 70 change(cur);tmp[tot]=cur; 71 tot++; 72 dfs(cur+1); 73 tot--; 74 change(cur); 75 76 } 77 int main() 78 { 79 #ifndef ONLINE_JUDGE 80 freopen("in","r",stdin); 81 #endif 82 int len=0; 83 for(int i=0;i<4;i++) 84 { 85 scanf("%s",str[i]); 86 for(int j=0;j<4;j++) 87 if(str[i][j] == 'b')a[++len]=1; 88 else a[++len]=0; 89 } 90 tot=0;ans=(1<<16); 91 dfs(1); 92 if(ans != (1<<16))printf("%d\n",ans); 93 else puts("Impossible"); 94 return 0; 95 }