LeetCode 3823: Reverse Letters Then Special Characters in a String ·

Table of Contents

Meta Data
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Difficulty: easy First Attempt: 2026-01-31

Submissions: Accepted (first try)

Approach
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Reverse the letters back into the letter positions, then reverse the special characters back into theirs.

Each pass is “reverse while skipping the other kind of character” with converging pointers: each side skips ahead to a character of the kind being processed, they swap, and both move inward. The two passes touch disjoint positions, so their order does not matter.

My Solution
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class Solution:
    def reverseByType(self, s: str) -> str:
        chars = list(s)

        def reverse(keep) -> None:
            left, right = 0, len(chars) - 1
            while left < right:
                while left < right and not keep(chars[left]):
                    left += 1
                while left < right and not keep(chars[right]):
                    right -= 1
                chars[left], chars[right] = chars[right], chars[left]
                left += 1
                right -= 1

        reverse(str.isalpha)
        reverse(lambda ch: not ch.isalpha())
        return "".join(chars)

My original wrote the two passes out separately with opposite conditions. Here “which kind to process” is a parameter, so both passes share one function.

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

Mistakes to Avoid
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Inner skipping loops without a left < right check: the pointers can cross the middle and swap the wrong positions.
Modifying the string directly: convert it to a list first.

Interview Explanation
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I run two converging-pointer reversals: the first swaps only letters, with each pointer skipping non-letters; the second swaps only special characters. The passes touch disjoint positions, so they can be done separately. One reversal function takes the predicate as a parameter, and the total is O(n).