Solving DSA Problems. LeetCode 58: Length of Last Word

Problem statement

Given a string s consisting of words and spaces, return the length of the last word in the string.

A word is a maximal substring consisting of non-space characters only.

Example 1

Input: s = "Hello World"
Output: 5
Explanation: The last word is "World" with length 5.

Example 2

Input: s = " fly me to the moon "
Output: 4
Explanation: The last word is "moon" with length 4.

Example 3

Input: s = "luffy is still joyboy"
Output: 6
Explanation: The last word is "joyboy" with length 6.

Constraints

• 1 <= s.length <= $10^4$
• s consists of only English letters and spaces ' '.
• There will be at least one word in s.

Default Code

int lengthOfLastWord(char* s) {}

---

Solution

One would think that this is an easy task, where you get the whole string, parse it, and get the length of the last word. It is correct, however, considering we are solving in C, there is a better way to do it. However, we are going to do both approaches. Even though it does not really matter in this case, considering input is read and parsed before passing it to the given function, we will explore both options.

Approach 1: Parse string with strtok

So, that's what comes to the mind first: parse by space, and get the last word and its length! So, let's do that. Remember that, the parsing function in C is strtok:

int lengthOfLastWord(char* s)
{
    char* token = strtok(s, " ");
    char *last_word = NULL;

    while (token != NULL)
    {
        last_word = token;
        token = strtok(NULL, " ");
    }

    return strlen(last_word);
}

If we submit this:

Okay, submission is accepted, but stats can be better. Let's try another approach.

Approach 2: Parse string with sscanf

With sscanf one would ask? You see, scanf and relative functions get string values quite different by nature: it scans the string to the value TILL WHITESPACE. So, we can use it to get words by whole.

int lengthOfLastWord(char* s)
{
    char word[10001];
    int len = 0;

    while(sscanf(s, "%s%n", word, &len) > 0)
        s += len;

    return strlen(word);
}

So, what did we do:

1. Used %n to get the number of scanned characters
2. Move the pointer to the next word after each scan.
3. Continue till the EOF, and return the length of the last scanned string.

Simple, as that! So, if LeetCode allowed us, we could have done this without even storing the string, parsing right from stdin. This would have increased both the memory and runtime of the program. If we submit this solution:

We got the best time in runtime! That's perfect, however, we still kind of use a lot of memory (mostly because of word[10000]). So, what if we drop all tools and try a hardcore loop search?

Approach 3: Loop search

Of course, we can do everything the old-fashioned way:

int lengthOfLastWord(char* s)
{
    int count = 0;
    int i = strlen(s) - 1;

    // we can check this without checking `i`
    // because there is always at least one word
    while (s[i] == ' ')
        --i;

    while (i >= 0 && s[i] != ' ')
        ++count, --i;

    return count;
}

And if we submit:

Accepted with the best time, and better memory usage! Yay!

You can access the code here. Feel free to contribute your solution in a different language!