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"]
解題思路
1.將字元陣列s反轉（原地操作，不可使用額外陣列）。
2.解法概念：雙指標交換
一個指標從左邊開始 (left = 0)。
一個指標從右邊開始 (right = s.length - 1)。
3.每次交換兩個字元後，left++、right--。
停止條件：
當left >= right時，反轉完成。