Reverse Words in a String III
Problem
Given a string s, reverse the order of characters in each word within a sentence while preserving whitespace and initial word order.
- 1 ≤ s.length ≤ 5 * 10⁴
- s contains printable ASCII characters.
- s does not contain leading or trailing spaces.
- All words are separated by a single space.
Example
s = "Let's take LeetCode contest""s'teL ekat edoCteeL tsetnoc"The algorithm splits the input string into words: ["Let's", "take", "LeetCode", "contest"]. It reverses each word individually: "Let's" → "s'teL", "take" → "ekat", "LeetCode" → "edoCteeL", "contest" → "tsetnoc". Finally, it joins the reversed words with spaces, preserving the original word order and spacing.
Approach
Straightforward Solution
A brute-force approach would iterate through the string character by character, manually reversing each word in place, which is more complex and error-prone.
Core Observation
Each word can be treated as an independent unit for reversal because the problem requires reversing characters within words but preserving word order and spacing.
Path to Optimal
Splitting the string into words using built-in methods simplifies the problem by isolating each word. Reversing each word individually and then joining them back preserves the original word order and spacing, achieving the goal cleanly and efficiently.
Optimal Approach
Use the built-in split() method to separate the string into words, reverse each word using slicing, and then join the reversed words with spaces. This approach is concise, readable, and runs in O(n) time where n is the length of the string.
Time
O(n)
Splitting the string and joining the words each take O(n) time, and reversing each word is O(k) where k is the word length; summing over all words yields O(n) total.
Space
O(n)
The auxiliary space is O(n) due to storing the list of words and the reversed words before joining; this is necessary to hold the output string.
Pattern Spotlight
String Manipulation (Word-by-Word Transformation)
When a problem requires modifying each word independently while preserving overall word order and spacing, split the string into words, transform each word separately, and then rejoin them to maintain structure.
Solution
| 1 | class Solution:
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| 2 | def reverseWords(self, s: str) -> str:
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| 3 | words = s.split()
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| 4 | for i in range(len(words)):
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| 5 | words[i] = words[i][::-1]
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| 6 | return " ".join(words) |
Step-by-Step Solution
Split Input String into Words
| 3 | words = s.split()
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Objective
To separate the input string into individual words for independent processing.
Key Insight
Using the built-in split() method efficiently isolates words by whitespace, enabling focused transformations on each word without worrying about spaces or word boundaries during reversal.
Interview Quick-Check
Core Logic
Splitting the string into words creates discrete units that can be reversed independently, preserving the original word order.
Common Pitfalls & Bugs
Forgetting that split() removes whitespace can lead to incorrect spacing if not handled properly during the join step.
Reverse Characters in Each Word
To transform each word by reversing its characters while keeping the word's position intact.
Join Reversed Words into Final String
To reconstruct the final string by concatenating reversed words with spaces, preserving original word order and spacing.
2 more steps with full analysis available on Pro.
Line Analysis
This solution has 1 Critical line interviewers watch for.
words[i] = words[i][::-1]Reverse the characters of the current word using slicing.
Slicing with [::-1] is a concise and efficient way to reverse strings in Python, avoiding manual character swaps or additional data structures.
Full line-by-line criticality + rationale for all 4 lines available on Pro.
Test Your Understanding
Why is it more efficient and less error-prone to split the string into words before reversing each word, rather than reversing characters in place?
See the answer with Pro.
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