LeetCode 3794: Reverse String Prefix ·

Table of Contents

Meta Data
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Difficulty: easy First Attempt: 2026-01-03

Submissions: Accepted (slicing) → Runtime Error ×2 → Accepted (two pointers, 2026-01-24)

Approach
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Reverse the first k characters. Python slicing does it in one line: s[:k][::-1] + s[k:].

My Attempts: rewriting with two pointers for practice
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When I came back in late January to rewrite it with two pointers, two Runtime Errors followed:

right = len(k) - 1: k is an integer and has no len; it should be k - 1.
After that, s[left], s[right] = s[right], s[left] failed: Python strings cannot be modified, so convert with list(s) first and "".join(...) at the end.

Solution
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class Solution:
    def reversePrefix(self, s: str, k: int) -> str:
        chars = list(s)
        left, right = 0, k - 1
        while left < right:
            chars[left], chars[right] = chars[right], chars[left]
            left += 1
            right -= 1
        return "".join(chars)

Complexity
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Time O(n), space O(n) (strings are immutable, so a new one is always built).

Mistakes to Avoid
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Assigning to characters of a string: Python strings are immutable.
Calling len on an integer.

Interview Explanation
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Python strings are immutable, so I convert to a list and swap with two pointers from index 0 and index k - 1 toward the middle, then join back into a string. Slicing, s[:k][::-1] + s[k:], also works; both are O(n).