Write a function that reverses a string. The input string is given as an array of characters `s`.
You must do this by modifying the input array **in-place** with O(1) extra memory.
**Example 1:**
```
Input: s = ["h","e","l","l","o"]
Output: ["o","l","l","e","h"]
```
**Example 2:**
```
Input: s = ["H","a","n","n","a","h"]
Output: ["h","a","n","n","a","H"]
```