541. Reverse String II-反转字符串 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:

• The string consists of lower English letters only.
• Length of the given string and k will in the range [1, 10000]

Solution in python:

class Solution:
    def reverseStr(self, s: str, k: int) -> str:
        length = len(s)
        i = 0
        result = ""
        while i < length:
            result += (s[i:i+k])[::-1]
            result += s[i+k:i+2*k]
            i += 2*k
        return result