def intToChar(x): return chr(ord('a')+x)
allsubstrings = []
for mask in range(26):
arr = [mask]
ss = ''.join(intToChar(x) for x in arr)
allsubstrings.append(ss)
for mask in range(26 ** 2):
arr = []
for _ in range(2):
arr.append(mask % 26)
mask //= 26
arr.reverse()
ss = ''.join(intToChar(x) for x in arr)
allsubstrings.append(ss)
for mask in range(26 ** 3):
arr = []
for _ in range(3):
arr.append(mask % 26)
mask //= 26
arr.reverse()
ss = ''.join(intToChar(x) for x in arr)
allsubstrings.append(ss)
allsubstrings = allsubstrings[: 3000]
def main():
t = int(input())
allans = []
for _ in range(t):
n = int(input())
s = input()
substrings = set()
for i in range(n):
substrings.add(s[i])
if i + 1 < n:
substrings.add(s[i: i + 2])
if i + 2 < n:
substrings.add(s[i: i + 3])
for ss in allsubstrings:
if ss not in substrings:
ans = ss
break
allans.append(ans)
multiLineArrayPrint(allans)
return
import sys
input=lambda: sys.stdin.readline().rstrip("\r\n")
def oneLineArrayPrint(arr):
print(' '.join([str(x) for x in arr]))
def multiLineArrayPrint(arr):
print('\n'.join([str(x) for x in arr]))
def multiLineArrayOfArraysPrint(arr):
print('\n'.join([' '.join([str(x) for x in y]) for y in arr]))
def readIntArr():
return [int(x) for x in input().split()]
def makeArr(defaultValFactory,dimensionArr): dv=defaultValFactory;da=dimensionArr
if len(da)==1:return [dv() for _ in range(da[0])]
else:return [makeArr(dv,da[1:]) for _ in range(da[0])]
def queryInteractive(a, b, c):
print('? {} {} {}'.format(a, b, c))
sys.stdout.flush()
return int(input())
def answerInteractive(x1, x2):
print('! {} {}'.format(x1, x2))
sys.stdout.flush()
inf=float('inf')
from math import gcd,floor,ceil
import math
for _abc in range(1):
main()#include<bits/stdc++.h>
using namespace std;
#define int long long
// #define ll long long
#define f(i,n) for(int i=0;i<n;i++)
#define fi(i,x,n) for(int i=x;i<n;i++)
#define pii pair<int,int>
#define vi vector<int>
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define rof(i,a,b) for(int i=(a);i>=(b);i--)
#define S second
#define F first
#define is insert
#define sz(a) (int)(a.size())
void solve() {
int n; cin >> n;
string s;
cin >> s;
bool ans = false;
int count[26] = {0};
for (int i = 0; i < n; i++) {
count[s[i] - 'a'] = 1;
}
for (int i = 0; i < 26; i++) {
if (count[i] == 0) {
cout << (char)('a' + i) << '\n';
return;
}
}
{
set<string>st;
for (int i = 0; i < n - 1; i++) {
string temp = "";
temp += s[i];
temp += s[i + 1];
st.insert(temp);
}
for (int i = 0; i < 26; i++) {
for (int j = 0; j < 26; j++) {
string temp = "";
temp += (char)('a' + i);
temp += (char)('a' + j);
if (st.find(temp) == st.end()) {
cout << temp << "\n";
return;
}
}
}
}
{
set<string>st;
for (int i = 0; i < n - 2; i++) {
string temp = "";
temp += s[i];
temp += s[i + 1];
temp += s[i + 2];
st.insert(temp);
}
for (int i = 0; i < 26; i++) {
for (int j = 0; j < 26; j++) {
string temp = "a";
temp += (char)('a' + i);
temp += (char)('a' + j);
if (st.find(temp) == st.end()) {
cout << temp << "\n";
return;
}
}
}
}
}
int32_t main() {
#ifndef ONLINE_JUDGE
// for getting input from input.txt
freopen("input.txt", "r", stdin);
// for writing output to output.txt
freopen("output.txt", "w", stdout);
#endif
int t = 1;
cin >> t;
while (t--)
{
solve();
//cout << "\n";
}
}1204A - BowWow and the Timetable | 508B - Anton and currency you all know |
1672A - Log Chopping | 300A - Array |
48D - Permutations | 677C - Vanya and Label |
1583B - Omkar and Heavenly Tree | 1703C - Cypher |
1511C - Yet Another Card Deck | 1698A - XOR Mixup |
1702E - Split Into Two Sets | 1703B - ICPC Balloons |
1702F - Equate Multisets | 1700A - Optimal Path |
665C - Simple Strings | 1708A - Difference Operations |
1703E - Mirror Grid | 1042A - Benches |
1676B - Equal Candies | 1705B - Mark the Dust Sweeper |
1711A - Perfect Permutation | 1701B - Permutation |
1692A - Marathon | 1066A - Vova and Train |
169B - Replacing Digits | 171D - Broken checker |
380C - Sereja and Brackets | 1281B - Azamon Web Services |
1702A - Round Down the Price | 1681C - Double Sort |