#include <bits/stdc++.h>
#pragma GCC optimize("-O3")
using namespace std;
const long long N = 1e6 + 5;
void pairsort(long long a[], long long b[], long long n) {
  pair<long long, long long> pairt[n];
  for (long long i = 0; i < n; i++) {
    pairt[i].first = a[i];
    pairt[i].second = b[i];
  }
  sort(pairt, pairt + n);
  for (long long i = 0; i < n; i++) {
    a[i] = pairt[i].first;
    b[i] = pairt[i].second;
  }
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long isPrime(long long n) {
  if (n < 2) return 0;
  if (n < 4) return 1;
  if (n % 2 == 0 or n % 3 == 0) return 0;
  for (long long i = 5; i * i <= n; i += 6)
    if (n % i == 0 or n % (i + 2) == 0) return 0;
  return 1;
}
long long C(long long n, long long r) {
  if (r > n - r) r = n - r;
  long long ans = 1;
  for (long long i = 1; i <= r; i++) {
    ans *= n - r + i;
    ans /= i;
  }
  return ans;
}
long long mod = 1e9 + 7;
long long modexpo(long long x, long long p) {
  long long res = 1;
  x = x % mod;
  while (p) {
    if (p % 2) res = res * x;
    p >>= 1;
    x = x * x % mod;
    res %= mod;
  }
  return res;
}
long long t, n, ind1 = -1, ind2 = -1, k;
string s1, s2;
map<char, set<long long>> M1, M2;
vector<pair<long long, long long>> v;
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  cin >> t;
  while (t--) {
    cin >> n >> s1 >> s2, ind1 = -1, ind2 = -1, k = 0, M1.clear(), M2.clear(),
                          v.clear();
    for (long long i = 0; i < n; i++) M1[s1[i]].insert(i), M2[s2[i]].insert(i);
    for (long long i = 0; i < n; i++) {
      if (s1[i] != s2[i]) {
        M1[s1[i]].erase(i);
        M2[s2[i]].erase(i);
        if (M2[s2[i]].size()) {
          long long x = *M2[s2[i]].begin();
          v.push_back({i, x});
          M2[s2[i]].erase(M2[s2[i]].begin());
          M1[s1[i]].erase(i);
          swap(s1[i], s2[x]);
          M2[s2[x]].insert(x);
        } else if (i + 1 < n && M1[s2[i]].size()) {
          long long x1 = *M1[s2[i]].begin();
          M1[s2[i]].erase(x1);
          M2[s2[i + 1]].erase(i + 1);
          swap(s1[x1], s2[i + 1]);
          v.push_back({x1, i + 1});
          M1[s1[x1]].insert(x1);
          swap(s1[i], s2[i + 1]);
          v.push_back({i, i + 1});
          M2[s2[i + 1]].insert(i + 1);
        } else
          k = 1;
      }
      M1[s1[i]].erase(i);
      M2[s2[i]].erase(i);
    }
    if (k)
      cout << "No"
           << "\n";
    else {
      cout << "Yes"
           << "\n";
      cout << v.size() << "\n";
      for (long long i = 0; i < v.size(); i++)
        cout << v[i].first + 1 << " " << v[i].second + 1 << "\n";
    }
  }
  return 0;
}
