From LeetCode 541. 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 leave the other as original.

Solution

class Solution:
    def reverseStr(self, s: str, k: int) -> str:
        """
        1. 使用range(start, end, step)来确定需要调换的初始位置
        2. 对于字符串s = 'abc'，如果使用s[0:999] ===> 'abc'。字符串末尾如果超过最大长度，则会返回至字符串最后一个值，这个特性可以避免一些边界条件的处理。
        3. 用切片整体替换，而不是一个个替换.
        """
        def reverse_substring(text):
            left, right = 0, len(text) - 1
            while left < right:
                text[left], text[right] = text[right], text[left]
                left += 1
                right -= 1
            return text
        
        res = list(s)

        for cur in range(0, len(s), 2 * k):
            res[cur: cur + k] = reverse_substring(res[cur: cur + k])
        
        return ''.join(res)