#include <bits/stdc++.h>
#define ll long long int
#define ull unsigned long long int
using namespace std;
void solve()
{
ll n;
cin >> n;
string a, b;
cin >> a;
cin >> b;
vector<char> ans(2 * n);
sort(a.begin(), a.end());
sort(b.rbegin(), b.rend());
ll ansstart = 0, ansend = (2 * n) - 1;
ll astart = 0, aend = n - 1;
ll bstart = 0, bend = n - 1;
for (ll i = 0; i < 2 * n; i++)
{
if (i % 2 == 0)
{
if (bstart <= bend && a[astart] >= b[bstart])
{
ans[ansend] = a[aend];
ansend--;
aend--;
}
else
{
ans[ansstart] = a[astart];
astart++;
ansstart++;
}
}
else
{
if (astart <= aend && a[astart] < b[bstart])
{
ans[ansstart] = b[bstart];
ansstart++;
bstart++;
}
else
{
ans[ansend] = b[bend];
ansend--;
bend--;
}
}
}
for (ll i = 0; i < 2 * n; i++)
{
cout << ans[i];
}
cout << "\n";
}
int main()
{
ll t;
cin >> t;
while (t--)
{
solve();
}
}
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