Given a string S, return the "reversed" string where all characters that are not a letter stay in the same place, and all letters reverse their positions.
Example 1:
Input: "ab-cd"
Output: "dc-ba"
Example 2:
Input: "a-bC-dEf-ghIj"
Output: "j-Ih-gfE-dCba"
Example 3:
Input: "Test1ng-Leet=code-Q!"
Output: "Qedo1ct-eeLg=ntse-T!"
Note:
S.length <= 100
33 <= S[i].ASCIIcode <= 122 
S doesn't contain \ or "
class Solution:
    def reverseOnlyLetters(self, S: str) -> str:
        S = list(S)
        start = 0
        end = len(S) - 1
        while start <= end:
            while not S[start].isalpha() and start < end:
                start += 1
            while not S[end].isalpha() and start < end:
                end -= 1
            S[start], S[end] = S[end], S[start]
            start += 1
            end -= 1
        return ''.join(S)
                

class Solution:
    def reverseOnlyLetters(self, S: str) -> str:
        S = list(S)
        start = 0
        end = len(S) - 1
        while start <= end:
            while not S[start].isalpha() and start < end:
                start += 1
            while not S[end].isalpha() and start < end:
                end -= 1
            S[start], S[end] = S[end], S[start]
            start += 1
            end -= 1
        return ''.join(S)
                
