class Solution:
    def reverseString(self, s: List[str]) -> None:
        """
        Do not return anything, modify s in-place instead.
        """
        s[:] = s[::-1]

Recursive

Here is an example. Let's implement recursive function helper which receives two pointers, left and right, as arguments.

Base case: if left >= right, do nothing.
Otherwise, swap s[left] and s[right] and call helper(left + 1, right - 1).

To solve the problem, call helper function passing the head and tail indexes as arguments: return helper(0, len(s) - 1).

Let's see this in action.

Let's say the string is "hello".

["h","e","l","l","o"]

First call:

helper(0, 4)
s[left], s[right] = s[right], s[left]

>>> x[0], x[4] = x[4], x[0]
>>> x
['o', 'e', 'l', 'l', 'h']

helper(1, 3)

Breaks when left pointer is more than right pointer.