#include<bits/stdc++.h>
using namespace std;
 
#define LL long long int
#define LD long double
 
LL gcd(LL a, LL b) { return a%b==0?b:gcd(b,a%b); }
LL lcm(LL a, LL b) { return (a*b)/gcd(a,b); }
LL mod(LL a, LL b) { return a>b?a-b:b-a; }
LL sq(LL x) { return x*x; }
LL cu(LL x) { return x*x*x; }
LL expo(LL a, LL n) {
	if(n == 0) return 1;
	LL k = expo(a,n/2);
	return n%2 == 0 ? sq(k) : a*sq(k);
}
LL modmul(LL a, LL b, LL mod) {
    LL res = 0; a %= mod;
    while (b) { if (b & 1) res = (res + a) % mod; a = (2 * a) % mod; b >>= 1; }
    return res;
}
LL vsum(vector<LL> v, LL n, LL loc) { if(loc==-1) return 0; for(LL i=1;i<n;i++) v[i] += v[i-1]; return v[loc]; }
LL vsum(vector<int> v, LL n, LL loc) { if(loc==-1) return 0; for(LL i=1;i<n;i++) v[i] += v[i-1]; return v[loc]; }
LL vfind(vector<LL> &v, LL n, LL k) { for(LL i=0;i<n;i++) if(v[i]==k) return i; return -1; }
LL vfind(vector<int> &v, LL n, LL k) { for(LL i=0;i<n;i++) if(v[i]==k) return i; return -1; }
LL vmin(vector<LL> &v, LL n) { LL loc=0; for(LL i=1;i<n;i++) if(v[i]<v[loc]) loc=i; return loc;}
LL vmin(vector<int> &v, LL n) { LL loc=0; for(LL i=1;i<n;i++) if(v[i]<v[loc]) loc=i; return loc;}
LL vmax(vector<LL> &v, LL n) { LL loc=0; for(LL i=1;i<n;i++) if(v[i]>v[loc]) loc=i; return loc;}
LL vmax(vector<int> &v, LL n) { LL loc=0; for(LL i=1;i<n;i++) if(v[i]>v[loc]) loc=i; return loc;}
void vin(vector<LL> &v, LL n) { for(LL i=0;i<n;i++) cin >> v[i]; }
void vout(vector<LL> &v, LL n) { for(LL i=0;i<n;i++) cout << v[i] << " "; }
void vin(vector<int> &v, LL n) { for(LL i=0;i<n;i++) cin >> v[i]; }
void vout(vector<int> &v, LL n) { for(LL i=0;i<n;i++) cout << v[i] << " "; }
void vout(vector<char> &v, LL n) { for(LL i=0;i<n;i++) cout << v[i] << " "; }
void vout(vector<bool> &v, LL n) { for(LL i=0;i<n;i++) cout << v[i] << " "; }

LL MAX = 1e6;
LL N;
vector<bool> temp;
set<int> seive;

//-------------------------------------------------------------------------------------------------\\

void makeprime() {
	for(int i=2;sq(i)<=N;i++) {
    	if(temp[i] == false) {
        	for(int j=2*i;j<=N;j+=i) {
            	if(temp[j] == false) temp[j] = true;
            }
        }
    }
	for(int i=2;i<=N;i++) if(temp[i] == false) seive.insert(i);
}

//|\\|//|\\|//|\\|//|\\|//|\\|//|\\|//|\\|//|\\|//|\\|//|\\|//|\\|//|\\|//|\\|//|\\|//|\\|//|\\|//|\\

bool compair(pair<LL,pair<LL,LL>> &p1, pair<LL,pair<LL,LL>> &p2) { return p1.first > p2.first; }

LL fact(int n) {
	LL res = 1;
	while(n>0) {
		res *= n;
		n--;
	}
	return res;
}

LL ncr(int n, int r) {
	LL num = 1, den =1;
	LL tr = r+1;
	while(tr <= n) { num *= tr; tr++; }
	int k = n-r;
	while(k > 0) { den *= k; k--; }
	return num/den;
}

// -----------------------------------------------------------------------------------------------------------------


// z: 122

void inc(string &s) {
	int n = s.size();
	int t;
	if(n == 1) {
		t = (int)s[0];
		if(t<122) {
			t++;
			s[n-1] = (char)t;
		}
		else {
			s = "aa";
		}
	}
	else if(n==2) {
		t = (int)s[1];
		if(t<122) {
			t++;
			s[1] = (char)t;
		}
		else {
			s[1] = 'a';
			t = (int)s[0];
			if(t<122) {
				t++;
				s[0] = (char)t;
			}
			else {
				s = "aaa";
			}
		}
	}
	else {
		t = (int)s[2];
		if(t<122) {
			t++;
			s[2] = (char)t;
		}
		else {
			s[2] = 'a';
			t = (int)s[1];
			if(t<122) {
				t++;
				s[1] = (char)t;
			}
			else {
				s[1] = 'a';
				t = (int)s[0];
				t++;
				s[0] = (char)t;
			}
		}
	}
}

void solve() {
	int n; cin >> n;
	string s; cin >> s;
	string t = "a";
	while(true) {
		auto k = s.find(t);
		if(k == string::npos) { cout << t << "\n"; break; }
		else inc(t);
	}
}

int main() {

#ifndef ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
	auto START = std::chrono::high_resolution_clock::now();
#endif

// handle input here...

	// makeprime(); //Use this to generate prime numbers, also don't forget to set N.

bool testcases = true;
	if(testcases) { int T; cin >> T; while(T--) solve(); }
	else solve();

#ifndef ONLINE_JUDGE
	auto STOP = std::chrono::high_resolution_clock::now();
	auto elapsed = std::chrono::duration_cast<std::chrono::microseconds>(STOP-START);
	printf("\n\nRuntime: %.3f ms.\n",elapsed.count()*1e-3);
#endif
	return 0;
}