0058 - Length of Last Word

Problem Link

https://leetcode.com/problems/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.

Approach 1: Looping Backwards

We can simply initialize a pointer to start at the end of the string, iterate backwards until we find a non-space character, then start our algorithm. Which will be to iterate backwards until the next space character, incrementing a variable to track the length of the word.

Time Complexity: $O(n)$, where $n$ is the length of the string, $s$. In the worst case, we will have to iterate over the whole string.

Space Complexity: $O(1)$. We only need to track a pointer variable, and a variable to track the length of the word.

Python

Written by @ColeB2

class Solution:
    def lengthOfLastWord(self, s: str) -> int:
        # initialize i as our pointer at the end of s.
        i = len(s) - 1
        # initialize length, as the length of our last word
        length = 0
        # reach First word.
        while i >= 0 and s[i] == ' ':
            i -= 1
        # count length first word
        while i >= 0 and s[i] != ' ':
            i -= 1
            length += 1
        return length