左旋转字符串

public static String (String str, int n) {
       if (str == null || str.length() == 0 || n < 0) {
           return "";
       }
       StringBuilder sb = new StringBuilder(str);
       int len = str.length();
       n = n % len;
       sb.append(str);
       return sb.substring(n, len + n);
   }