LeetCode 151: Reverse Words in a String

Meta Data

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Difficulty: medium First Attempt: 2022-11-12

• Submissions: Accepted ×3 → Runtime Error → Wrong Answer ×5 → Accepted

Approach

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str.split() with no argument splits on runs of whitespace of any length and drops leading and trailing spaces, giving exactly the word list. Reverse it and join with single spaces. With an O(1)-space requirement on a character array, reverse everything and then each word (Two Pointers Template 1).

My Attempts

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It passed in 2022. A 2026 rewrite used split(" "), which produces empty strings for consecutive spaces: "b b" returned "b b" (still two spaces) and "b " returned " b"; one version also forgot the separating space, turning "a b" into "ba".

Solution

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class Solution:
    def reverseWords(self, s: str) -> str:
        return " ".join(reversed(s.split()))

Complexity

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

Mistakes to Avoid

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• split(" ") produces empty strings for consecutive spaces; split() does not.

Interview Explanation

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Calling split() with no argument yields the words with all extra spaces handled, and I join them reversed with single spaces. Time O(n).