[String-easy] 541. Reverse String II inverting elements, and has the number of inversion interval

1. The title site

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

2. Title Description

3. subject to the effect

Given a string, and the string of interval k, k represents the number of inverted once every k, k elements and then re-reversal interval k elements.

4. Problem-solving ideas

Just follow the interval to reverse it. Then the interval k elements is not reversed by letting i * k complete each increment of 2.

5. AC Code

class Solution {
    public String reverseStr(String s, int k) {
        int length = s.length();
        char[] arr = s.toCharArray();
        for(int i = 0; i < length; i += 2*k) { // 主要是这个地方
            reverse(arr, i, k);
        }
        return String.valueOf(arr);
    }

    private void reverse(char[] s, int start, int k) {
        int index = start + k - 1;
        if(index > s.length - 1) index = s.length - 1;
        while(start < index) {
            char temp = s[start];
            s[start] = s[index];
            s[index] = temp;
            start ++; index --;
        }
    }
}

6. similar questions

. [1] 344 Reverse String title site: https://leetcode.com/problems/reverse-string/
[2] 541. Reverse String II site topic: https://leetcode.com/problems/reverse-string-ii /
[3] 557. Reverse Words in a String III site topic: https://leetcode.com/problems/reverse-words-in-a-string-iii/