541. Reverse String II

Edited by Charles

Easy · Python · String

Problem

Read the original problem on LeetCode.

The official page contains the problem statement, examples, and constraints.

Explanation, examples & other approaches

Core idea

This solution is based on the solution to 344. Reverse String We need to convert the string into a list of characters in the string.

Original study notes follow, including intermediate snippets and earlier attempts.

Note:

1. This solution is based on the solution to 344. Reverse String
2. We need to convert the string into a list of characters in the string.
3. Iterate the whole list every 2*k step and only operate reversing on first k elements.

Study record

Field	Value
Attempts	1
Mastery	❚
The Latest Attempt	January 6, 2026

Python solution

class Solution:
    def reverseStr(self, s: str, k: int) -> str:
        n = len(s)
        result = list(s)

        for i in range(0, n, 2 * k):
            result[i: i + k] = self.reverseSubstr(result[i: i + k])

        return ''.join(result)

    def reverseSubstr(self, s):
        l = len(s)
        left, right = 0, l - 1

        while left < right:
            s[left], s[right] = s[right], s[left]
            left += 1
            right -= 1

        return s

---

Back to Day 7 String I · All Algorithm notes