LeetCode 541 - Reverse String II

https://leetcode.com/problems/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]

https://discuss.leetcode.com/topic/82626/java-concise-solution

    public String reverseStr(String s, int k) {
        char[] arr = s.toCharArray();
        int n = arr.length;
        int i = 0;
        while(i < n) {
            int j = Math.min(i + k - 1, n - 1);
            swap(arr, i, j);
            i += 2 * k;
        }
        return String.valueOf(arr);
    }
    private void swap(char[] arr, int l, int r) {
        while (l < r) {
            char temp = arr[l];
            arr[l++] = arr[r];
            arr[r--] = temp;
        }
    }

https://discuss.leetcode.com/topic/82627/c-java-clean-code

    public String reverseStr(String s, int k) {
        char[] ca = s.toCharArray();
        for (int left = 0; left < ca.length; left += 2 * k) {
            for (int i = left, j = Math.min(left + k - 1, ca.length - 1); i < j; i++, j--) {
                char tmp = ca[i];
                ca[i] = ca[j];
                ca[j] = tmp;
            }
        }
        return new String(ca);
    }

X.
https://discuss.leetcode.com/topic/82570/verbose-java-solution-stringbuilder-s
.substring is O(n)

    public String reverseStr(String s, int k) {
        StringBuilder sb = new StringBuilder();
        
        int i = 0, j = 0;
        while (i < s.length()) {
            // first k
            j = i + k <= s.length() ? i + k : s.length();
            sb.append((new StringBuilder(s.substring(i, j))).reverse().toString());
            
            if (j >= s.length()) break;
            
            // second k
            i = j;
            j = i + k <= s.length() ? i + k : s.length();
            sb.append(s.substring(i, j));
            
            i = j;
        }
        
        return sb.toString();
    }