def find_first_letter(input_str):
for i in range(len(input_str)):
if input_str[i] == 'R':
return 'R' if i & 1 == 0 else 'B'
if input_str[i] == 'B':
return 'B' if i & 1 == 0 else 'R'
return 'B'
t = int(input())
for iter in range(t):
n = int(input())
input_str = input()
result_str = ''
if input_str[0] == '?':
if n > 1:
result_str += find_first_letter(input_str)
input_str = result_str + input_str[1:]
else:
print('B')
continue
else:
result_str += input_str[0]
for i in range(1, n):
if input_str[i] in ['R', 'B']:
result_str += input_str[i]
continue
if result_str[i - 1] == 'B':
result_str += 'R'
elif result_str[i - 1] == 'R':
result_str += 'B'
print(result_str)
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define py cout<<"YES"<<endl
#define pn cout<<"NO"<<endl
#define e cout<<"=\n"
#define g cout<<">\n"
#define l cout<<"<\n"
#define Pans cout<<"ans=="<<" ";
bool is_lleger(float k) {return floor(k) == k;}
int main()
{
ll t;
cin>>t;
while(t--){
int n;
cin>>n;
string s;
cin>>s;
int i=0;
while(s[i]=='?'){
i++;
}
if(i==n){
char c='B';
for(int i=0;i<n;i++){
s[i]=c;
if(c=='R')c='B';
else c='R';
}
}
else{
for(int j=i+1;j<n;j++){
if(s[j]=='?'){
if(s[j-1]=='R') s[j]='B';
else if(s[j-1]=='B') s[j]='R';
}
}
for(i=i-1;i>=0;i--){
if(s[i]=='?'){
if(s[i+1]=='R') s[i]='B';
else if(s[i+1]=='B') s[i]='R';
}
}
}
// Pans;
cout<<s<<endl;
}
return 0;
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