要求

编写一个函数，其作用是将输入的字符串反转过来。输入字符串以字符数组 s 的形式给出。
不要给另外的数组分配额外的空间，你必须原地修改输入数组、使用 O(1) 的额外空间解决这一问题。

示例

输入：s = ["h","e","l","l","o"]
输出：["o","l","l","e","h"]

输入：s = ["H","a","n","n","a","h"]
输出：["h","a","n","n","a","H"]

提示

1 <= s.length <= 105
s[i] 都是 ASCII 码表中的可打印字符

解答

要求本地修改，设置双指针从头到尾一次变换，当指针相碰时结束调换。

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