0541. Reverse String II

https://leetcode.com/problems/reverse-string-ii

Description

Given a string s and an integer k, reverse the first k characters for every 2k characters counting from the start of the string.

If there are fewer 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 1:

**Input:** s = "abcdefg", k = 2
**Output:** "bacdfeg"

Example 2:

**Input:** s = "abcd", k = 2
**Output:** "bacd"

Constraints:

• 1 <= s.length <= 104

• s consists of only lowercase English letters.

• 1 <= k <= 104

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class Solution {
    public String reverseStr(String s, int k) {
        // edge cases
        if (s.length() <= 1 || k <= 1) return s;

        char[] chars = s.toCharArray();
        int l = 0, i = 0;
        do {
            i += 2 * k;
            int r = Math.min(l + k - 1, s.length() - 1); // elegant
            reverse(chars, l, r);
            l = i;
        } while (i < s.length());

        return String.valueOf(chars);
    }

    private void reverse(char[] chars, int left, int right) {
        while (left < right) {
            char tmp = chars[left];
            chars[left++] = chars[right];
            chars[right--] = tmp;
        }
    }
}