LeetCode 541: Reverse String II

Meta Data

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Difficulty: easy First Attempt: 2022-08-25

• Submissions: Wrong Answer ×2 → Runtime Error → Wrong Answer ×2 → Runtime Error → Accepted → Runtime Error ×2 → Wrong Answer ×2 → Accepted

Approach

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Step from 0 by 2k and reverse [i, i + k) (Two Pointers Template 1). When fewer than k characters remain, all of them are reversed, and the slice a[i:i + k] truncates at the end, which matches exactly.

My Attempts

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The 2022 versions reversed only the first block: "cddebd", k = 2 should be "dcdedb"; it returned "dcdebd". In 2026 I reversed only when i + k <= n, skipping a final block shorter than k: "cd", k = 4 should be "dc"; it returned "cd".

Solution

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class Solution:
    def reverseStr(self, s: str, k: int) -> str:
        a = list(s)
        for i in range(0, len(a), 2 * k):
            a[i:i + k] = reversed(a[i:i + k])
        return "".join(a)

Complexity

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Time O(n), space O(n).

Mistakes to Avoid

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• Not just the first block: every block of 2k.
• Fewer than k remaining are reversed too.

Interview Explanation

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Stepping by 2k, I reverse the first k of each block, and slicing automatically reverses only what remains at the end. Time O(n).