Doubt in snakproc problem

general
1

why am i getting wrong answer?
running perfectly in IDLE

n = int(input())
if n == 0:
    print('Invalid')
    exit
for i in range(n):
    s_num = int(input())
    st = input()[:s_num]
    ls = []
    for i in st:
        if i != '.':
            if i == 'H' and i in ls:
                break
            elif i == 'H' and i not in ls:
                ls += [i]        
            elif len(ls) == 1 and i == 'T':
                ls.pop()`Preformatted text`         
    if ls == []:
        print('Valid')
    else:
        print('Invalid')