### maxLength 
from typing import *
def f_gold(arr: List[str]) -> int:
    def ones_count(x):
        c = 0
        while x:
            x &= x - 1
            c += 1
        return c
    ans = 0
    masks = [0]
    for s in arr:
        mask = 0
        for ch in s:
            ch = ord(ch) - ord('a')
            if (mask >> ch) & 1 == 1:
                mask = 0
                break
            mask |= 1 << ch
        if mask == 0:
            continue
        for m in masks:
            if m & mask == 0:
                masks.append(m | mask)
                ans = max(ans, ones_count(m | mask))
    return ans