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# 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`

## ac

```java
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;
        }
    }
}
```