Leetcode 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]

Code (Java):

class Solution {
    public String reverseStr(String s, int k) {
        if (k <= 1) {
            return s;
        }

        char[] charArr = s.toCharArray();
        int start = 0;
        int end = 0;

        while (start < charArr.length) {
            end = Math.min(start + k - 1, charArr.length - 1);
            reverse(charArr, start, end);

            start += k * 2;
        }

        return new String(charArr);
    }

    private void reverse(char[] charArr, int start, int end) {
        while (start < end) {
            char temp = charArr[start];
            charArr[start] = charArr[end];
            charArr[end] = temp;

            start++;
            end--;
        }
    }
}