917. 反转字符串（isalpha()）

解答一： 

def reverseOnlyLetters(self, S):
    i, j = 0, len(S) - 1
    S = list(S)
    while i < j:
        while i < j and not S[i].isalpha(): i += 1
        while i < j and not S[j].isalpha(): j -= 1
        S[i], S[j] = S[j], S[i]
        i, j = i + 1, j - 1
    return "".join(S) 

解答二：

class Solution:
    def reverseOnlyLetters(self, S):
        """
        :type S: str
        :rtype: str
        """
        newS = ""
        for i, ch in enumerate(S):
            # 如果字符串至少有一个字符并且所有字符都是字母则返回 True,否则返回 False
            if ch.isalpha(): # 检测字符串是否只由字母组成
                newS += ch
        # print(newS) # TestngLeetcodeQ
        newS = newS[::-1]  # reverse string

        # feed reversed strings back into
        index = 0
        r = ""
        for i, ch in enumerate(S):
            if ch.isalpha():
                r += newS[index]
                index += 1
            else:
                r += ch
        return r