B.cpp

#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>
#include <map>
#include <set>
#include <queue>
#include <cstdio>

using namespace std;

void pcase(int t) {
  cout << "Case #" << t << ": ";
}

int N, M;
bool cn[110][110];
bool vis[110];

bool dfs(int x, int snk) {
  if(x == snk) return true;
  if(vis[x]) return false;
  vis[x] = true;
  for(int i = 0; i <= snk; i++) {
    if(cn[x][i] && dfs(i, snk)) {
      swap(cn[x][i], cn[i][x]);
      return true;
    }
  }
  return false;
}

bool can(const string& A, const string& B) {
  int src = 2 * M;
  int snk = 2 * M + 1;

  int buck = N / M;
  memset(cn, 0, sizeof(cn));
  for(int i = 0; i < M; i++) {
    cn[src][i] = cn[M + i][snk] = true;
    for(int j = 0; j < M; j++) {
      bool ok = true;
      for(int k = 0; ok && k < buck; k++) {
        ok = A[i * buck + k] == '?' || B[j * buck + k] == '?' ||
             A[i * buck + k] == B[j * buck + k];
      }
      cn[i][M + j] = ok;
    }
  }

  for(int i = 0; i < M; i++) {
    memset(vis, 0, sizeof(vis));
    if(!dfs(src, snk)) return false;
  }
  return true;
}

int main() {
  int T; cin >> T;
  for(int t = 1; t <= T; t++) {
    pcase(t);

    cin >> M;
    string K1, K2; cin >> K1 >> K2;
    N = K1.size();

    if(!can(K1, K2)) {
      cout << "IMPOSSIBLE" << endl;
      continue;
    }
    for(int i = 0; i < N; i++) {
      if(K1[i] != '?') continue;
      for(char ch = 'a'; ; ch++) {
        K1[i] = ch;
        if(can(K1, K2)) break;
      }
    }
    cout << K1 << endl;
  }
  return 0;
}