541. Reverse String II

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.

class Solution {
    public String reverseStr(String s, int k) {
        StringBuilder sb = new StringBuilder (s);
        int len =  s.length();
        int count = len/(2*k);
        int remain = len%(2*k);
         if(count>0)
         {
              for(int i = 0 ;i<count;i++ )
           {
            int start = 2*k*i;
            int end = 2*k*i+k-1;
           reverse(sb,start,end);
           }
         }
        if(remain<k)
        {
            int start =2*k*count;
            int end =2*k*count+remain-1;
            reverse(sb,start,end);
        }
        else
        {
            int start =2*k*count;
            int end =2*k*count+k-1;
            reverse(sb,start,end);
        }
        return sb.toString();
    }
    private void reverse (StringBuilder sb,int start,int end)
    {
        while(start<=end)
        {
            char temp = sb.charAt(start);
            sb.setCharAt(start,sb.charAt(end));
            sb.setCharAt(end,temp);
            start++;
            end--;
        }
    }
}