#include <bits/stdc++.h>
using namespace std;
long long dj[] = {1, 0, -1, 0};
long long di[] = {0, 1, 0, -1};
long long fine(long long i, long long j, long long n, long long m) {
  return i >= 0 && i < n && j >= 0 && j < m;
}
long long gcd(long long a, long long b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long k;
  cin >> k;
  for (; k; k--) {
    long long n;
    cin >> n;
    string s, t;
    cin >> s >> t;
    map<char, queue<long long>> first, second;
    long long cnt = 0;
    for (long long i = 0; i < n; i++) {
      if (s[i] != t[i]) {
        first[s[i]].push(i);
        second[t[i]].push(i);
      }
    }
    long long f = 1;
    queue<pair<long long, long long>> ans;
    for (long long i = 0; i < n; i++) {
      if (s[i] != t[i]) {
        long long check = 0;
        for (long long j = i + 1; j < n; j++) {
          if (s[i] == s[j] && s[j] != t[j]) {
            ans.push({j, i});
            check = 1;
            swap(t[i], s[j]);
            break;
          }
        }
        if (!check) {
          for (long long j = i + 1; j < n; j++) {
            if (t[j] == s[i] && t[j] != s[j]) {
              ans.push({j, j});
              check = 1;
              swap(s[j], t[j]);
              i--;
              break;
            }
          }
        }
        if (!check) {
          f = 0;
          break;
        }
      }
    }
    if (f && ans.size() <= 2 * n) {
      cout << "Yes\n";
      cout << ans.size() << '\n';
      while (ans.size()) {
        cout << ans.front().first + 1 << " " << ans.front().second + 1 << '\n';
        ans.pop();
      }
    } else
      cout << "No\n";
  }
}
