反转字符串（简单）

class Solution {
    public void reverseString(char[] s) {
        if(s==null || s.length==0) return ;
        int slen=s.length-1;
        for(int i=0;i<=slen/2;i++){
            char ch=s[slen-i];
            s[slen-i]=s[i];
            s[i]=ch;
        }
    }
}