Given a string and an integer k, you need to reverse the first k characters for every 2k characters counting from the start of the string. If there are less than k characters left, reverse all of them. If there are less than 2k but greater than or equal to k characters, then reverse the first k characters and left the other as original.

Example:

Input: s = "abcdefg", k = 2
Output: "bacdfeg"

Restrictions:

1. The string consists of lower English letters only.
2. Length of the given string and k will in the range [1, 10000]

   import java.util.LinkedList;
   
   /**
    * @author zhangyu
    * @version V1.0
    * @ClassName: ReverseString2Test
    * @Description: Given a string and an integer k, you need to reverse the first k 
      characters for every 2k characters  counting from the start of the string. If there 
      are less than k characters left, reverse all of them.
    * If there are less than 2k but greater than or equal to k characters,
    * then reverse the first k characters and left the other as original.
    * @date 2018/10/14 15:34
    **/
   
   public class ReverseString2Test {
       public static void main(String[] args) {
           String s = "abcdefg";
           int key = 2;
           String newString = getReverseString2(s, 2);
           System.out.println(newString);
       }
   
       private static String getReverseString2(String s, int k) {
           //當s字串的長度小於k的值
           if (s.length() <= k) {
               return new StringBuffer(s).reverse().toString();
           }
           //當s字串的長度介於 k和2k之間
           if (s.length() > k && s.length() <= 2 * k) {
               String subString = s.substring(0, k);
               subString = new StringBuffer(subString).reverse().toString();
               return subString + s.substring(k, s.length());
           }
           //當k的字串的長度大於2k
           if (s.length() > 2 * k) {
               LinkedList<String> list = new LinkedList<>();
               StringBuffer sb = new StringBuffer();
               int number = s.length() / k;
               for (int i = 0; i < number; i++) {
                   list.add(s.substring(i * k, i * k + k));
               }
               if (number * k < s.length()) {
                   list.add(s.substring(number * k));
               }
               for (int index = 0; index < list.size(); index++) {
                   if (index % 2 == 0) {
                       String temp = new StringBuffer(list.get(index)).reverse().toString();
                       sb.append(temp);
                   } else {
                       sb.append(list.get(index));
                   }
               }
               return sb.toString();
           }
           return null;
       }
   }