1974B - Symmetric Encoding - CodeForces Solution

for _ in range(int(input())):
        n=int(input())
    s=input() 
    ss=list(s)
    ss.sort()
    ls=[]
    for i in range(n-1):
        if ss[i]!=ss[i+1]:
            ls.append(ss[i])
    ls.append(ss[n-1])    
    d=dict()
    i=0;j=len(ls)-1
    while i<=j:
        d[ls[i]]=ls[j]
        d[ls[j]]=ls[i]
        i+=1 
        j-=1
    ans=[]
    for a in s:
        ans.append(d[a])
    print(''.join(ans))