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

The optimal solution has a time complexity of O(n).

What data structures are used to solve Reverse Words in a String III?

This problem uses Two Pointers, String. Understanding these concepts is essential for solving this Easy problem efficiently.

What are the key insights for solving Reverse Words in a String III?

Insight 1: The core task is reversing individual words, not the entire string. Insight 2: The string can be split into words using the space character as a delimiter.

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Reverse Words in a String III | LeetCode 557

Q: Which companies ask LeetCode 557. Reverse Words in a String III in interviews?

This problem is frequently asked at BNY Mellon, Devtron, Salesforce, Walmart Labs, Wissen Technology and 1 more companies.

Easy

Two Pointers String

Time: O(n)Space: O(n)

Solution Code

Available in:

class Solution {
    public String reverseWords(String s) {
        String[] words = s.split(" ");
        StringBuilder sb = new StringBuilder();
        for (String word : words) {
            sb.append(new StringBuilder(word).reverse().toString()).append(" ");
        }
        return sb.toString().trim();
    }
}

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Problem Info

Description

Problem 557: Reverse Words in a String III - Detailed explanation and solution approaches.

Topics

Two Pointers String

Companies

BNY MellonDevtronSalesforceWalmart LabsWissen TechnologyYandex

Stats

Difficulty:Easy

Languages:4

Companies:6

Problem #:557

Problem Explanation

The problem asks us to reverse the order of characters within each word of a given string, while preserving the original word order and the spaces between them. For example, if the input is "Let's take LeetCode contest", the output should be "s'teL ekat edoCteeL tsetnoc". The input string consists of words separated by single spaces and contains only printable ASCII characters, without leading or trailing spaces.

💡

Think Before You Code

🤔 Ask Yourself

→What is the input? What output do I need?
→Can I solve a smaller version of this problem first?
→What patterns have I seen in similar problems?

✏️ Try This First

Before looking at the solution, try to solve it on paper with a simple example!

🚀 Try on LeetCode

Solution Approach

The solution involves splitting the input string into individual words, reversing each word, and then joining the reversed words back together with spaces. The specific implementation varies slightly depending on the programming language, but the core idea remains the same.

📋

Step-by-Step Solution

4 steps

1
Step 1: Split the input string into a list of words, using the space character as the delimiter.
2
Step 2: Iterate through the list of words.
3
Step 3: For each word, reverse the order of its characters.
4
Step 4: Join the reversed words back together with spaces to form the final output string.

💡

Pro Tip for Students

Try tracing through each step with a small example on paper before coding. Understanding "why" each step happens is more important than memorizing the code!

⚡

Complexity Analysis

⏱️

Time Complexity

O(n)

👍 Linear - Good! Visits each element once.

💾

Space Complexity

O(n)

📦 Linear space - Memory grows with input size.

📝

Why?

The time complexity is linear because we iterate through the entire string once to split it, reverse each word which in total is another iteration through the string, and then join the words back together. The space complexity is linear because, in most implementations, we create a new string to store the reversed words, whose size is proportional to the input string size.

📖New to Big O? Click to learn more

Big O Notation - Quick Guide

O(1)Best - Instant

O(log n)Excellent - Binary Search

O(n)Good - One loop

O(n log n)OK - Sorting

O(n²)Slow - Nested loops

O(2ⁿ)Very Slow - Recursion

Alternative Approaches

In-place reversal using two pointers (C/C++ style)

Iterate through the string, identifying word boundaries (spaces). Use two pointers within each word to swap characters and reverse it directly in the original string.

Time: O(n)Space: O(1) if truly in-place (modifying the original string directly). If a copy is created during manipulation, then O(n).

Trade-offs: More efficient in terms of space, especially if the compiler can optimize in-place string modification, but more complex to implement than splitting and joining. Suited for languages where direct string manipulation is common and efficient.

Using regular expressions

Use regular expressions to identify each word and reverse it.

Time: O(n)Space: O(n)

Trade-offs: Can be concise, but often less efficient than iterative approaches. Regular expression compilation and matching can add overhead.

✨

Key Insights

1Insight 1: The core task is reversing individual words, not the entire string.
2Insight 2: The string can be split into words using the space character as a delimiter.
3Insight 3: After reversing each word, the words need to be joined back together with spaces.

📚

Key Terms Explained

For Students

Two Pointers

Using two indices to traverse data from different positions simultaneously.

String

A sequence of characters. Like a word or sentence in programming!

Edge Cases

Edge case 1: Empty string. The solution should handle an empty string gracefully, returning an empty string or an appropriate empty result.
Edge case 2: String with only one word. The solution should correctly reverse the single word and return it.
Edge case 3: String with multiple spaces between words. While the problem states that words are separated by single spaces, a robust solution could handle cases with multiple spaces (which split() will often handle correctly anyway).
Edge case 4: String with special characters. The code assumes ASCII characters. This case is already covered in constraints of the question.

⚠️

Common Mistakes to Avoid

✗Common mistake 1: Incorrectly handling spaces. Forgetting to preserve spaces between words or adding extra spaces.

✗Common mistake 2: Not handling edge cases like empty strings or single-word strings.

✗Common mistake 3: Modifying the original string directly without creating a copy when the problem requires returning a new string (or vice-versa, modifying copy when in-place modification is the goal).

✗Common mistake 4: Not handling special characters appropriately and making assumptions about string contents.

✅

Learning Tip

Every mistake is a learning opportunity! When you make an error, understand why it happened before moving on.

🌍

Where is This Used in Real Life?

💼Real-world scenario 1: Text processing and manipulation. Reversing words can be used in tasks such as creating mirror images of text or implementing simple obfuscation techniques.

🏦Real-world scenario 2: Data cleaning and transformation. In some data sets, fields might be stored with reversed words, and this algorithm can be used to correct the data.

💡Understanding real-world applications helps you see the "why" behind algorithms. It's not just about passing interviews—these concepts power the apps and websites you use every day!

Author's Notes

💡 Key Insight

The two-pointer technique is powerful for sorted arrays. The key insight is recognizing when elements can be processed from both ends simultaneously.

🎯 Interview Tip

Don't rush. Explain your thought process clearly. Easy problems are often about communication.

⚠️ Watch Out

The most common mistake is not handling the case when pointers meet. Always consider the termination condition.

Written by Saswata Mondal

Software Engineer • Algorithms Enthusiast • Interview Coach

👨‍💻

About the Author

👨‍💻

Saswata MondalAuthor400+ LC Problems

B.Tech CSE, IEM Kolkata | LeetCode Rating: 1672 (Top 15%)

Certified: IBM Java Developer, NPTEL Programming in Java

Last updated: June 18, 2024LeetCode Profile GitHub

This solution has been crafted based on my experience solving 400+ LeetCode problems. Each explanation is designed to help you understand the underlying concepts, not just memorize the code.

Community Tips & Insights

I was asked this exact problem at my Google interview. The interviewer wanted me to optimize space complexity.

@InterviewProJul 13

Try solving this without using extra space first. It's a great exercise for understanding the underlying pattern.

@CodeNinjaJul 6

In a real interview, always clarify constraints first. Ask about input size and value ranges.

@TechLeadJun 29

❓

Frequently Asked Questions

How do I avoid infinite loops with two pointers?

Always ensure your loop condition guarantees termination (typically left < right or slow != fast). Move at least one pointer in each iteration, and handle edge cases where pointers might not move (like skipping duplicates). Draw out pointer movement on paper first.

What is the sliding window technique for string problems?

Sliding window maintains a "window" of characters with two pointers. Expand the right pointer to include characters, shrink from left when window violates constraints. Useful for substring problems, anagrams, and longest/shortest substring with properties.

What should I learn from easy problems?

Easy problems introduce core patterns that appear in harder problems. Master basic operations (iteration, conditionals, simple data structures), recognize common patterns (counting, searching, basic transformations), and practice explaining your thought process clearly.

What is the importance of time and space complexity analysis?

Complexity analysis is crucial because: 1) Interviewers always ask about it, 2) It helps you choose between approaches, 3) It demonstrates CS fundamentals. Always state both time and space complexity, and be prepared to explain how you derived them.

What is the optimal approach for "Reverse Words in a String III"?

For this easy-level Two Pointers problem, the key is to identify the core pattern. Analyze the input constraints to determine acceptable time complexity. Consider whether string techniques can optimize the solution. Always handle edge cases and validate your approach with examples before coding.

🤖 Need help with this problem?

Reverse Words in a String III - Complete Solution Guide

Problem Statement

Detailed Explanation

Solution Approach

Step-by-Step Algorithm

Key Insights

Complexity Analysis

Topics

Companies

Reverse Words in a String III | LeetCode 557

Solution Code

Problem Info

Description

Topics

Companies

Stats

Problem Explanation

Think Before You Code

🤔 Ask Yourself

✏️ Try This First

Solution Approach

Step-by-Step Solution

Pro Tip for Students

Complexity Analysis

Time Complexity

Space Complexity

Why?

Big O Notation - Quick Guide

Alternative Approaches

In-place reversal using two pointers (C/C++ style)

Using regular expressions

Key Insights

Key Terms Explained

Two Pointers

String

Edge Cases

Common Mistakes to Avoid

Learning Tip

Where is This Used in Real Life?

Author's Notes

About the Author

Community Tips & Insights

Frequently Asked Questions

How do I avoid infinite loops with two pointers?

What is the sliding window technique for string problems?

What should I learn from easy problems?

What is the importance of time and space complexity analysis?

What is the optimal approach for "Reverse Words in a String III"?

Reverse Words in a String III | LeetCode 557

Solution Code

Problem Info

Description

Topics

Companies

Stats

Problem Explanation

Think Before You Code

🤔 Ask Yourself

✏️ Try This First

Solution Approach

Step-by-Step Solution

Pro Tip for Students

Complexity Analysis

Time Complexity

Space Complexity

Why?

Big O Notation - Quick Guide

Alternative Approaches

In-place reversal using two pointers (C/C++ style)

Using regular expressions

Key Insights

Key Terms Explained

Two Pointers

String

Edge Cases

Common Mistakes to Avoid

Learning Tip

Where is This Used in Real Life?

Author's Notes

About the Author