#include<bits/stdc++.h>   using namespace std;   #define mod        1000000007   #define pb         push_back   #define mp         make_pair   #define rep(n)     for(int i=0;i<n;i++)   #define repit(x)   for(auto it : x)   #define ll         long long   #define vi         vector<int>   #define vc         vector<char>   #define vs         vector<string>   #define mii        map<int ,int>   #define mci        map<char ,int>   /* bool prime[100001];    const int N=100000;    void sieve() {    int i,j;   for(i=2;i<=N;i++)   prime[i]=true;    prime[0]=prime[1]=false;   for(i=2;i*i<=N;i++){   if(prime[i]){   for(j=i*i;j<=N;j+=i)   prime[j]=false;   }   }   }*/   int gcd(int a,int b){   if(b==0)   return a;   else   return gcd(b,a%b);   }   void swap(int *x,int *y){   int temp=*x;   *x=*y;   *y=temp;   }   int main()   {          ios::sync_with_stdio(0);   cin.tie(0);   cout.tie(0);   int t;   cin>>t;   while(t--){   int i,n,c=0,j;   cin>>n;   string s,s2;   cin>>s;   map<string,int>m;   for(i=97;i<=122;i++){       string s1;       s1.push_back(i);       m.insert(make_pair(s1,0));   }   for(i=0;i<n;i++){       string s1;       s1.push_back(s[i]);       m[s1]++;   }   for(auto it=m.begin();it!=m.end();it++){       if(it->second==0){           c=1;           s2=it->first;           break;       }   }   if(c){       cout<<s2<<  n ;   }   else{       m.clear();       for(i=97;i<=122;i++){           for(j=97;j<=122;j++){               string s1;               s1.push_back(i);               s1.push_back(j);               m.insert(make_pair(s1,0));           }       }       for(i=0;i<n-1;i++){           string s1;           s1.push_back(s[i]);           s1.push_back(s[i+1]);           m[s1]++;       }       for(auto it=m.begin();it!=m.end();it++){           if(it->second==0){               c=1;               s2=it->first;               break;           }       }       if(c)       cout<<s2<<  n ;       else{           m.clear();           for(i=97;i<=122;i++){               for(j=97;j<=122;j++){                   for(int k=97;k<=122;k++){                       string s1;                       s1.push_back(i);                       s1.push_back(j);                       s1.push_back(k);                       m.insert(make_pair(s1,0));                   }               }           }           for(i=0;i<n-2;i++){               string s1;               s1.push_back(s[i]);               s1.push_back(s[i+1]);               s1.push_back(s[i+2]);               m[s1]++;                          }           for(auto it=m.begin();it!=m.end();it++){               if(it->second==0){                   c=1;                   s2=it->first;                   break;               }           }           cout<<s2<<  n ;       }   }   }   return 0;   }