class Solution:
    def reverseOnlyLetters(self, s: str) -> str:
        def is_letter(c):
            return "a" <= c <= "z" or "A" <= c <= "Z"

        N = len(s)
        ans = list(s)

        e = N - 1
        for b in range(N):
            if b > e:
                break
            if not is_letter(s[b]):
                continue
            while not is_letter(s[e]):
                e -= 1
            ans[b], ans[e] = ans[e], ans[b]
            e -= 1

        return "".join(ans)