def generate(N, prefix=""):
    if len(prefix) == N:
        print(prefix)
        return
    for ch in "ABC":
        generate(N, prefix + ch)

def generate_condition(N, prefix=""):
    if len(prefix) == N:
        print(prefix)
        return
    if not prefix.endswith('A'):
        generate(N, prefix + 'A')
    generate(N, prefix + 'B')
    generate(N, prefix + 'C')
    

# generate(5, "")
generate_condition(5, "")