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#include <cstdio>
#include <cstring>
#include <algorithm>
#include <queue>
#include <cctype>
using namespace std;
typedef long long LL;
const int maxn = 33;
char s[maxn][maxn], ans[maxn];
bool vis[maxn];
bool g[maxn][maxn];
int d[maxn], cnt;
int L[maxn], R[maxn], U[maxn], D[maxn];
void proc(char c, int x) {
if (isalpha(c) && c - 'A' != x) {
g[x][c - 'A'] = true;
}
}
void build(int x) {
for (int i = L[x]; i <= R[x]; ++i) {
proc(s[U[x]][i], x);
proc(s[D[x]][i], x);
}
for (int i = U[x]; i <= D[x]; ++i) {
proc(s[i][L[x]], x);
proc(s[i][R[x]], x);
}
}
void dfs(int dep) {
if (dep == cnt) {
ans[dep] = 0;
puts(ans);
return;
}
for (int i = 0; i < maxn; ++i) {
if (vis[i] && d[i] == 0) {
vis[i] = false;
ans[dep] = 'A' + i;
for (int j = 0; j < maxn; ++j) {
if (g[i][j]) {
--d[j];
}
}
dfs(dep + 1);
vis[i] = true;
for (int j = 0; j < maxn; ++j) {
if (g[i][j]) {
++d[j];
}
}
}
}
}
int main() {
freopen("frameup.in", "r", stdin);
freopen("frameup.out", "w", stdout);
int n, m;
scanf("%d%d", &n, &m);
memset(L, 0x3f, sizeof L);
memset(U, 0x3f, sizeof U);
for (int i = 0; i < n; ++i) {
scanf("%s", s[i]);
for (int j = 0; j < m; ++j) {
if (isalpha(s[i][j])) {
int c = s[i][j] - 'A';
vis[c] = true;
L[c] = min(L[c], j);
R[c] = max(R[c], j);
U[c] = min(U[c], i);
D[c] = max(D[c], i);
}
}
}
for (int i = 0; i < maxn; ++i) {
if (vis[i]) {
++cnt;
build(i);
}
}
for (int i = 0; i < maxn; ++i) {
for (int j = 0; j < maxn; ++j) {
if (g[i][j] && i != j) {
++d[j];
}
}
}
dfs(0);
return 0;
}
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