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Length of Last Word – Solution & Complexity

Interview solutions
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Length of Last Word

Solution Walkthrough

  1. Step 1: Understand What Counts as a Word

    • A word is any maximal run of non-space characters.
    • Leading, trailing, and repeated spaces should not count as part of a word.
    • If the string contains only spaces, the answer is 0.

  2. Step 2: Simple Split-Based Idea

    • One straightforward approach is to split the string on whitespace.
    • The last token is the last word, and its length is the answer.
    • This is easy to read, but it creates a list of all words.
    def length_of_last_word(s):
    words = s.split()
    if not words:
        return 0
    return len(words[-1])

  3. Step 3: Scan from the End

    • The last word is closest to the end of the string.
    • First skip trailing spaces.
    • Then count characters until you reach another space or the beginning of the string.
    i = len(s) - 1
while i >= 0 and s[i] == " ":
    i -= 1

  4. Step 4: Count the Last Word Characters

    • After trailing spaces are skipped, i points at the last character of the last word.
    • Move left while characters are not spaces, increasing a counter.
    • If there was no word, the counter stays at 0.
    length = 0
while i >= 0 and s[i] != " ":
    length += 1
    i -= 1

  5. Step 5: Complete Solution and Complexity

    • The final solution uses the end scan to avoid storing extra words.
    • Time complexity is O(n) in the worst case.
    • Space complexity is O(1).
    def length_of_last_word(s: str) -> int:
    i = len(s) - 1

    while i >= 0 and s[i] == " ":
        i -= 1

    length = 0
    while i >= 0 and s[i] != " ":
        length += 1
        i -= 1

    return length

FAQ

See also

Practice Length of Last Word 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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