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.

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]

class Solution {
    public String reverseStr(String s, int k) {
        int n = s.length();
        StringBuilder sb = new StringBuilder();
        
        for(int i = 0; i < n; ) {
            if(n - i < k) {
                sb.append(rev(s.substring(i, n)));
                return sb.toString();
            }
            else if(n - i >= k && n - i < 2*k) { 
                sb.append(rev(s.substring(i, i+k))).append(s.substring(i+k));
                return sb.toString();
            }
            else {
                sb.append(rev(s.substring(i, i+k))).append(s.substring(i+k, i+2*k));
                i+=2*k;
            }
        }
        return sb.toString();
    }
    public String rev(String s) {
        return new StringBuilder(s).reverse().toString();
    }
}