How do you solve Reverse Words in a String?

Split the string into words using s.split(), which returns a list of words ignoring extra spaces. Reverse this list in-place using list.reverse(), then join the reversed words with a single space using ' '.join(words). This approach is concise, efficient, and leverages Python's optimized string methods.

Reverse Words in a String

Q: What is the time complexity of Reverse Words in a String?

O(n). Splitting the string scans all characters once, reversing the list is O(w) where w is the number of words (w ≤ n), and joining the words is O(n). Overall linear in the length of the input string.

Q: What pattern does Reverse Words in a String use?

The Pattern: String Manipulation (Word Splitting and Reversal) The Heuristic: When reversing word order in a string, use built-in split to tokenize words cleanly, reverse the token list, and join with a single space to handle spacing and ordering elegantly.

Medium Two Pointers

Problem

Given a string s, return a string with the words in reverse order, where words are defined as sequences of non-space characters separated by spaces.

1 ≤ s.length ≤ 10⁴
s contains English letters (upper-case and lower-case), digits, and spaces ' '
There is at least one word in s

Example

Input: s = "the sky is blue"

Output: "blue is sky the"

The straightforward approach is to split the string into words by spaces, reverse the order of these words, and then join them back with a single space. For example, splitting "the sky is blue" yields ["the", "sky", "is", "blue"]. Reversing this list gives ["blue", "is", "sky", "the"]. Joining with spaces produces "blue is sky the".

Approach

Straightforward Solution

A naive approach might manually parse the string character by character to extract words and then reverse them. This is error-prone and verbose.

Core Observation

The problem reduces to reversing the order of words in a string, where words are separated by spaces. The key is to correctly identify words and handle multiple spaces and leading/trailing spaces.

Path to Optimal

Preview

Using built-in string methods simplifies the problem. The split() method automatically handles multiple spaces and trims leading/trailing spaces by returning only the words…

Full step-by-step walkthrough on Pro →

Optimal Approach

Preview

Split the string into words using s.split(), which returns a list of words ignoring extra spaces…

Full step-by-step walkthrough on Pro →

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Time

O(n)

Splitting the string scans all characters once, reversing the list is O(w) where w is the number of words (w ≤ n), and joining the words is O(n). Overall linear in the length of the input string.

Space

O(n)

The split operation creates a list of words that in total contain all characters of the input string, so auxiliary space is proportional to input size.

Pattern Spotlight

String Manipulation (Word Splitting and Reversal)

When reversing word order in a string, use built-in split to tokenize words cleanly, reverse the token list, and join with a single space to handle spacing and ordering elegantly.

Solution

Python

1class Solution:
  def reverseWords(self, s: str) -> str:
      words = s.split()
      words.reverse()
      return " ".join(words)

Step-by-Step Solution

Split Input String into Words

3	`words = s.split()`

Objective

To extract all words from the input string, ignoring extra spaces.

Key Insight

The built-in split() method splits the string on whitespace and automatically discards empty strings caused by multiple spaces or leading/trailing spaces. This yields a clean list of words without manual parsing.

Interview Quick-Check

Core Logic

Using s.split() returns a list of words by splitting on any whitespace and ignoring empty tokens, which simplifies word extraction.

Common Pitfalls & Bugs

Manually splitting by ' ' without handling multiple spaces can lead to empty strings in the list, causing incorrect output.

Reverse the List of Words In-Place

To reverse the order of the extracted words efficiently.

Join Reversed Words into Output String

To concatenate the reversed words into a single string separated by spaces.

2 more steps with full analysis available on Pro.

Line Analysis

This solution has 1 Critical line interviewers watch for.

Line 3 Critical

words = s.split()

Split the input string into a list of words by whitespace.

This line leverages Python's built-in split() method, which automatically handles multiple spaces and trims leading/trailing spaces, producing a clean list of words essential for correct reversal.

Full line-by-line criticality + rationale for all 3 lines available on Pro.

Test Your Understanding

Why is using s.split() preferable to manually parsing the string for words?

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1	`class Solution:`
2	`def reverseWords(self, s: str) -> str:`
3	`words = s.split()`
4	`words.reverse()`
5	`return " ".join(words)`