Letter Game

input/code.cpp: In function 'int get_enc(std::vector<int>&)':
input/code.cpp:9:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < v.size(); ++i) {
                 ~~^~~~~~~~~~
input/code.cpp: In function 'int f(int)':
input/code.cpp:65:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < que.size(); ++i) {
                 ~~^~~~~~~~~~~~
input/code.cpp:72:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < que.size(); ++i) {
                 ~~^~~~~~~~~~~~
input/code.cpp:77:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i = 0; i < v.size(); ++i) {
                  ~~^~~~~~~~~~
input/code.cpp:83:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i = 0; i + 1 < v.size(); ++i) {
                  ~~~~~~^~~~~~~~~~
in...

#include <bits/stdc++.h>
using namespace std;
const int MN = 1e7;
//A = 0
//B = 1
//. = 2
int get_enc(vector<int> & v) {
	int res = 0;
	for(int i = 0; i < v.size(); ++i) {
		res *= 3;
		res += v[i];
	}
	return res;
}
vector<int> get_vector(int x, int len) {
	vector<int> v(len);
	for(int i = 0; i < len; ++i) {
		v[len-i-1] = x % 3;
		x /= 3;
	}
	return v;
}
int pow(int a, int b) {
	int res = 1;
	for(int i = 0; i < b; ++i) {
		res *= a;
	}
	return res;
}
int visited[MN];
int prv[MN];
int f(int tot_len) {
	if(tot_len <= 2) {
		cout<<"FAIL\n";
		return 0;
	}
	int chars = tot_len-2;
	vector<int> v(tot_len);
	vector<int> que;
	for(int zeros = 0; zeros <= chars; ++zeros) {
		for(int e_pos = 0; e_pos+1 < tot_len; ++e_pos) {
			int c_pos = 0;
			for(int j = 0; j < tot_len; ++j) {
				if(j == e_pos || j == e_pos + 1) {
					v[j] = 2;
				}
				else {
					if(c_pos < zeros) {
						v[j] = 0;
					}
					else {
						v[j] = 1;
					}
					++c_pos;
				}
			}
			//for(auto x: v) cout<<"AB."[x]<<' ';
			//cout<<'\n';
			que.push_back(get_enc(v));
		}
	}
	//cout<<'\n';
	memset(visited, -1, sizeof visited);
 
	for(int i = 0; i < que.size(); ++i) {
		visited[que[i]] = 0;
		prv[que[i]] = que[i];
		vector<int> v = get_vector(que[i], tot_len);
		//for(auto x: v) cout<<"AB."[x]<<' ';
		//cout<<'\n';
	}
	for(int i = 0; i < que.size(); ++i) {
		vector<int> v = get_vector(que[i], tot_len);
		int dist = visited[que[i]];
		int prev_enc = que[i];
		int em_pos = 0;
		for(int i = 0; i < v.size(); ++i) {
			if(v[i] == 2) {
				em_pos = i;
				break;
			}
		}
		for(int i = 0; i + 1 < v.size(); ++i) {
			if(v[i] != 2 && v[i+1] != 2) {
				swap(v[i], v[em_pos]);
				swap(v[i+1], v[em_pos+1]);
				int enc = get_enc(v);
				if(visited[enc] == -1) {
					prv[enc] = prev_enc;
					visited[enc] = dist+1;
					que.push_back(enc);
				}
				swap(v[i], v[em_pos]);
				swap(v[i+1], v[em_pos+1]);
			}
		}
	}
	/*
	int lim = pow(3, tot_len);
	pair<int, int> ma = {0, 0};
	for(int i = 0; i < lim; ++i) {
		vector<int> v = get_vector(i, tot_len);
		int empty_cnt = 0;
		int a_cnt = 0;
		int b_cnt = 0;
		for(int i = 0; i < v.size(); ++i) {
			if(v[i] == 2) ++empty_cnt;
			if(v[i] == 0) ++a_cnt;
			if(v[i] == 1) ++b_cnt;
		}
		if(empty_cnt != 2 || a_cnt < 2 || b_cnt < 2) continue;
		int ok = 1;
		for(int i = 0; i < v.size(); ++i) {
			if(v[i] == 2 && v[i+1] != 2) {
				ok = 0;
			}
			if(v[i] == 2) break;
		}
		if(!ok) continue;
		if(visited[i] == -1) {
			cout<<"FAIL\n";
			for(auto x: v) cout<<"AB."[x];
			cout<<'\n';
		}
		ma = max(ma, {visited[i], i});
	}
	cout<<ma.first<<endl;
	vector<int> v2 = get_vector(ma.second, tot_len);
	for(auto x: v2) cout<<"AB."[x];
	cout<<'\n';
	*/
}
 
vector<string> ans;
 
void move(string& str, int& a, int b) {
	swap(str[a], str[b]);
	swap(str[a+1], str[b+1]);
	ans.push_back(str);
	a = b;
}
 
int main() {
	int n;
	cin>>n;
	n *= 2;
	string s;
	cin>>s;
	if(n == 2) {
		cout<<0<<'\n';
		return 0;
	}
 
	int aas = 0;
	int bbs = 0;
	int dots = -1;
	for (int i = 0; i < s.size(); ++i) {
		char c = s[i];
		if (c == 'A') ++aas;
		if (c == 'B') ++bbs;
		if (c == '.' && dots == -1) dots = i;
	}
 
	int a = 0;
	int b = n - 1;
	
	while(true) {
		if(b - a+1 < 10) break;
		char t = (aas >= 4 ? 'A' : 'B');
		int i1 = (t == 'B' ? -1 : b + 1);
		if(t == 'A') {
			if(dots == a+1) {
				move(s, dots, dots+2);
				move(s, dots, a);
			}
			else if(dots >= a+2) {
				move(s, dots, a);
			}
			for (int i = a; i <= b; ++i) {
				if (s[i] == t) {
					if (t == 'A' && i < i1) i1 = i;
				}
			}	
			move(s, dots, i1);
			++a;
			--aas;
		}
		if(t == 'B') {
			if(dots == b-2) {
				move(s, dots, dots-2);
				move(s, dots, b-1);
			}
			else if(dots <= b-3) {
				move(s, dots, b-1);
			}
			for (int i = a; i <= b; ++i) {
				if (s[i] == t) {
					if (t == 'B' && i1 < i) i1 = i;
				}
			}	
			move(s, dots, i1-1);
			--b;
			--bbs;
		}
	}
	vector<int> v;
	for(int i = a; i <= b; ++i) {
		if(s[i] == 'A') v.push_back(0);
		if(s[i] == 'B') v.push_back(1);
		if(s[i] == '.') v.push_back(2);
	}
	f(b-a+1);
	int pos = get_enc(v);
	if(visited[pos] == -1) {
		cout<<-1<<'\n';
		return 0;
	}
	vector<int> states;
	while(1) {
		vector<int> v2 = get_vector(pos, b-a+1);
		/*
		for(auto x: v2) cout<<"AB."[x];
		cout<<'\n';
		*/
		int next_pos = prv[pos];
		//cout<<next_pos<<'\n';
		if(next_pos == pos) break;
		states.push_back(next_pos);
		pos = next_pos;
	}
	for(int i = 0; i < states.size(); ++i) {
		string s2;
		for(int j = 0; j < a; ++j) s2.push_back('A');
		vector<int> v = get_vector(states[i], b-a+1);
		for(auto x: v) s2.push_back("AB."[x]);
		for(int j = b+1; j < n; ++j) s2.push_back('B');
		ans.push_back(s2);
	}
	cout<<ans.size()<<'\n';
	for(int i = 0; i < ans.size(); ++i) {
		cout<<ans[i]<<'\n';
	}
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
	
 
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED