反轉字符串

因爲要在原字符串上該，因此採用雙指針法：

class Solution:
    def reverseString(self, s: List[str]) -> None:
        """
        Do not return anything, modify s in-place instead.
        """
        
        len_s = len(s)
        if len_s == 0:
            return
        start = 0
        stop = len_s - 1
        
        while(stop > start):
            t = s[stop]
            s[stop] = s[start]
            s[start] = t
            stop -= 1
            start += 1