反转字符串

示例 1：
输入：["h","e","l","l","o"]
输出：["o","l","l","e","h"]

示例 2：
输入：["H","a","n","n","a","h"]
输出：["h","a","n","n","a","H"]

class Solution {
    public static void reverseString(char [] s) {
        int n = s.length;
        for(int i=0;i<n/2;i++){
            // 头尾互换
            swap(s,i,n-i-1);
        }
    }
    public static void swap(char [] s,int a, int b) {
        char t = s[a];
        s[a] = s[b];
        s[b] = t;
    }
}