牛牛的串串

t=int(input())

for i in range(t):

n=int(input())

s=input().strip()

count =[0]*26;

for c in s:

count[ord(c)-ord('a')]+=1;

nums = [num for num in count if num!=0]

nums.sort();

is_beautiful = True;

for i in range(1,len(nums)):

if nums[i]-nums[i-1]!=1:

is_beautiful=False;

break;

if is_beautiful:

print("YES");

else:

print("NO");