1737. Change Minimum Characters to Satisfy One of Three Conditions

class Solution:
    def minCharacters(self, a: str, b: str) -> int:
        def f(cnt1, cnt2):
            for i in range(1, 26):
                t = sum(cnt1[i:]) + sum(cnt2[:i])
                nonlocal res
                res = min(res, t)

        m, n = len(a), len(b)
        cnt1 = [0] * 26
        cnt2 = [0] * 26
        for c in a:
            cnt1[ord(c) - ord('a')] += 1
        for c in b:
            cnt2[ord(c) - ord('a')] += 1
        res = m + n
        for c1, c2 in zip(cnt1, cnt2):
            res = min(res, m + n - c1 - c2)
        f(cnt1, cnt2)
        f(cnt2, cnt1)
        return res