#include <bits/stdc++.h>
using namespace std;
using ll = long long;
constexpr ll mod = ll(1e9 + 7);
template <typename A, typename B>
string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string& s) { return '"' + s + '"'; }
string to_string(const char c) { return string(1, c); }
string to_string(const char* s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto& x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cerr << '\n'; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int TT = 1;
  cin >> TT;
  while (TT--) {
    ll n;
    cin >> n;
    string a, b;
    cin >> a >> b;
    bool good = true;
    vector<pair<int, int>> ans;
    for (int i = 0; i < n; ++i) {
      if (a[i] == b[i]) continue;
      good = false;
      for (int j = i + 1; j < n; ++j) {
        if (a[j] == a[i]) {
          ans.push_back({j, i});
          swap(a[j], b[i]);
          good = true;
          break;
        }
      }
      if (!good) {
        for (int j = i + 1; j < n; ++j) {
          if (b[j] == a[i]) {
            ans.push_back({j, j});
            ans.push_back({j, i});
            swap(a[j], b[j]);
            swap(a[j], b[i]);
            good = true;
            break;
          }
        }
      }
      if (!good) break;
    }
    if (good) {
      cout << "Yes" << '\n';
      cout << ans.size() << '\n';
      for (auto& [aa, bb] : ans) {
        cout << (aa + 1) << " " << (bb + 1) << '\n';
      }
    } else {
      cout << "No" << '\n';
    }
  }
}
