[LeetCode] 344.Reverse String

这道题非常简单，就是双指针问题的基础版，时间复杂度是O(N),空间复杂度是O(1)

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