def calculate(n, b):
#   if b.count("_") == 0 and compare(b):
				# find if exists any single letter
    if b.count("_") == 0 and len(re.sub(r'((.)\2+)', "", b)) != 0:
        return "NO"
    for a in set(b):
        if a != "_" and b.count(a) == 1:
            return "NO"
    return "YES"
'''
Thanks to reddeadya, this part didn't work for "XXBBXX"
def compare(b):
    if len(b) == 1:
        return True
    for x in set(b):
        if x * b.count(x) != b[b.index(x):b.index(x) + b.count(x)]:
            return True
    return False
'''
for _ in range(int(input())):
    n = int(input())
    b = input()
    print(calculate(n, b))