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";
}
}236B - Easy Number Challenge | 275A - Lights Out |
147A - Punctuation | 253A - Boys and Girls |
1327E - Count The Blocks | 984A - Game |
12B - Correct Solution | 1355B - Young Explorers |
485A - Factory | 628A - Tennis Tournament |
1436B - Prime Square | 1707B - Difference Array |
1422C - Bargain | 1611F - ATM and Students |
660A - Co-prime Array | 1692F - 3SUM |
1470A - Strange Birthday Party | 190D - Non-Secret Cypher |
1721B - Deadly Laser | 1721C - Min-Max Array Transformation |
1721A - Image | 1180C - Valeriy and Deque |
557A - Ilya and Diplomas | 1037D - Valid BFS |
1144F - Graph Without Long Directed Paths | 1228A - Distinct Digits |
355B - Vasya and Public Transport | 1230A - Dawid and Bags of Candies |
1530A - Binary Decimal | 1472D - Even-Odd Game |