写一个方法，实现字符串的反转，如：输入abc，输出cba

   public static String reverse1(String s){
        int length = s.length();
        StringBuffer result = new StringBuffer();
        for(int i=length-1;i>=0;i--){
            result.append(s.charAt(i));
        }
        
        return result.toString();
    }
    public static String reverse2(String s){
        int length = s.length();
        String result = "";
        for(int i=length-1;i>=0;i--){
            result += s.substring(i,i+1);
        }
        
        return result;
    }
    public static String reverse3(String s){
        char[] chars = s.toCharArray();
        int length = chars.length;
        int mid = length/2;
        char temp;
        for(int i = 0; i<mid; i++){
            temp = chars[i];
            chars[i] = chars[length-i-1];
            chars[length-i-1] = temp;
        }
        
        return String.valueOf(chars);
    }