Length of Last Word

How long is the last word? Words are separated by spaces, and the string may have trailing ones.

lengthOfLastWord("Hello World")                    // 5
lengthOfLastWord("   fly me   to   the moon  ")    // 4

The standard library answer

func lengthOfLastWord(s string) int {
    var splits = strings.Split(strings.TrimSpace(s), " ")
    return len(splits[len(splits)-1])
}

TrimSpace removes the trailing spaces that make the naive version wrong, and after that the last field is the last word. Correct, including the awkward case of runs of spaces in the middle — Split yields empty strings for those, but never as the final element once the string is trimmed.

The one-pass version

Split allocates a slice of every word to use exactly one of them. Walking backwards uses none:

func lengthOfLastWord(s string) int {
    i := len(s) - 1

    for i >= 0 && s[i] == ' ' {   // skip trailing spaces
        i--
    }

    length := 0
    for i >= 0 && s[i] != ' ' {   // count the word
        length++
        i--
    }

    return length
}

$O(n)$ worst case, $O(1)$ space, and it usually touches only the last few characters rather than all of them.

Worth noticing

This is the rare problem where the library version and the hand-written version are both defensible, and for opposite reasons. strings.Fields is what to write in production: shorter, obviously correct, and someone else maintains the edge cases. The backwards scan is what an interviewer is asking for: it demonstrates that you noticed the work being wasted.

Knowing which answer the question wants is part of the question.