#include <bits/stdc++.h>
using namespace std;
template <typename Tp>
void read(Tp &num) {
  num = 0;
  char c = getchar();
  long long f = 1;
  while (!isdigit(c)) c == '-' ? f = -1 : 1, c = getchar();
  while (isdigit(c)) num = (num << 3) + (num << 1) + (c ^ 48), c = getchar();
  num *= f;
}
template <typename Tp, typename... etc>
void read(Tp &num, etc &...more) {
  read(num);
  read(more...);
}
const long long maxn = 1e6 + 7;
long long t, n, a[maxn], b[maxn], ans1[maxn], cnt1, ans2[maxn], cnt2;
void sp(long long i, long long j) {
  long long r = a[j];
  a[j] = a[i];
  a[i] = r;
}
void fz(long long k) {
  for (register long long i = 1; i <= k / 2; ++i) sp(i, k - i + 1);
  for (register long long i = 1; i <= k; ++i) a[i] = a[i] ^ 1;
}
signed main() {
  read(t);
  while (t--) {
    read(n);
    cnt1 = 0;
    cnt2 = 0;
    char c;
    long long col1, col2;
    for (register long long i = 1; i <= n; ++i) {
      cin >> c;
      a[i] = c - '0';
    }
    for (register long long i = 1; i <= n; ++i) {
      cin >> c;
      b[i] = c - '0';
    }
    col1 = a[1];
    col2 = b[1];
    for (register long long i = 2; i <= n; ++i) {
      if (a[i] != col1) {
        ans1[++cnt1] = i - 1;
        col1 ^= 1;
      }
    }
    for (register long long i = 2; i <= n; ++i) {
      if (b[i] != col2) {
        ans2[++cnt2] = i - 1;
        col2 ^= 1;
      }
    }
    if (col1 != col2) ans1[++cnt1] = n;
    cout << cnt1 + cnt2 << endl;
    for (register long long i = 1; i <= cnt1; ++i) cout << ans1[i] << " ";
    for (register long long i = cnt2; i >= 1; --i) cout << ans2[i] << " ";
    cout << endl;
  }
  return 0;
}
