//My program assumes 0 based indexing
#include<bits/stdc++.h>
using namespace std;
int main()
{
    int T;
    cin>>T;
    while(T--)
    {
int N;             //Length of String str
cin>>N;            
string str;
cin>>str;          //String of length N and contains lowerCase Alphabets only
int maxlength = 0; //Initially assume 0 length if no subsequnece cant be selected
int freq[26]; //Frequency Array storing last visited position of lowercase alphabets 
for(int i = 0;i<26;i++)
{
freq[i] = -1; //Initialisation of frequency array to -1 to indicate no character has previously occured .0 is not used because 0 is used in indexing the string .
}
for(int i = 0;i<N;i++){
int code = str[i]-'a'; //mapped to 0,1..26 to indicate a,b...z
if(freq[code]!=-1)     //if already this character is present before 
{
maxlength = max(N-(i-freq[code]-1)-1,maxlength); //Considering N-(j-i) for all possible i,j where str[i]==str[j] and i!=j
}
freq[code] = i; //Updating last visited position of the lowercase alphabet
}
cout<<maxlength<<"\n"; //Outputting the maximum length possible of the sub-sub-sequences as answer 
}
}

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