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Reverse Words in a String – Solution & Complexity

Interview solutions
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Reverse Words in a String

Solution Walkthrough

  1. Step 1: Use whitespace tokenization first

    A direct approach is to split the string into words, which automatically drops extra spaces, then reverse the word list and join it back together with a single space.

    def reverse_words(s: str) -> str:
    return " ".join(reversed(s.split()))

  2. Step 2: Scan from the end with two pointers

    To avoid relying on a built-in split-on-whitespace helper, walk from right to left. Skip trailing spaces, mark the end of the current word, move left until the previous space, append that word, and repeat. Time: O(n). Space: O(n) for the output words/result.

    def reverse_words(s: str) -> str:
    words = []
    i = len(s) - 1

    while i >= 0:
        while i >= 0 and s[i] == ' ':
            i -= 1
        if i < 0:
            break

        j = i
        while j >= 0 and s[j] != ' ':
            j -= 1

        words.append(s[j + 1:i + 1])
        i = j - 1

    return " ".join(words)

FAQ

See also

Practice Reverse Words in a String online (run code in the browser)Alien Dictionary solution (hard)Count Vowels solution (easy)Decode String solution (medium)Decode Ways solution (medium)All coding interview problems
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