Reverse String

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 lessthan 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 asoriginal.

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]

public class Solution {
    public String reverseStr(String s, int k) {
        int n = s.length();
        StringBuilder result = new StringBuilder();
        
        for (int i = 0; i < n; ) {
            if (i + k > n) {
                // less than k chars left, reverse all
                result.append(reverseStr(s, i, n - 1));
                break;
            } else if (i + k <= n && i + 2 * k > n) {
                // k ~ 2k, reverse the first k chars, left the remaining as the same
                result.append(reverseStr(s, i, i + k - 1));
                result.append(s.substring(i + k));
                break;
            } else if (i + 2 * k <= n) {
                // more than 2k chars left, reverse the first k chars
                result.append(reverseStr(s, i, i + k - 1));
                result.append(s.substring(i + k, i + 2 * k));
                i += 2 * k;
            } 
        }
        return result.toString();
    }
    
    private StringBuilder reverseStr(String s, int begin, int end) {
        StringBuilder result = new StringBuilder();
        while (end >= begin) {
            result.append(s.charAt(end--));
        }
        return result;
    }
}