What is the time complexity of Rearrange Words in a Sentence?

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

What data structures are used to solve Rearrange Words in a Sentence?

This problem uses String, Sorting. Understanding these concepts is essential for solving this Medium problem efficiently.

What are the key insights for solving Rearrange Words in a Sentence?

Insight 1: Splitting the sentence into words is crucial for individual manipulation. Insight 2: Using a stable sorting algorithm (like Python's `sorted()` or implementing a stable sort in other languages) ensures the preservation of original word order when lengths are equal.

Which companies ask LeetCode 1451. Rearrange Words in a Sentence in interviews?

This problem is frequently asked at Expedia.

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Rearrange Words in a Sentence - LeetCode 1451 Solution

Rearrange Words in a Sentence - Complete Solution Guide

Rearrange Words in a Sentence is LeetCode problem 1451, a Medium level challenge. This complete guide provides step-by-step explanations, multiple solution approaches, and optimized code in python3, java, cpp, c.

Problem Statement

Given a sentence text (A sentence is a string of space-separated words) in the following format: First letter is in upper case. Each word in text are separated by a single space. Your task is to rearrange the words in text such that all words are rearranged in an increasing order of their lengths. If two words have the same length, arrange them in their original order. Return the new text following the format shown above. Example 1: Input: text = "Leetcode is cool" Output: "Is cool leetcode" Exp

Detailed Explanation

The problem requires you to rearrange the words in a given sentence such that the words are ordered by their lengths in ascending order. If two words have the same length, their original order in the sentence should be preserved. The input sentence starts with a capital letter, and each word is separated by a single space. The output sentence should also start with a capital letter.

Solution Approach

The solution involves splitting the input sentence into individual words, converting them to lowercase, sorting the words based on their lengths while maintaining the original order for words of the same length, joining the sorted words back into a sentence, and capitalizing the first letter of the resulting sentence.

Step-by-Step Algorithm

Step 1: Convert the input sentence to lowercase and split it into an array (or vector) of individual words using spaces as delimiters.
Step 2: Sort the array of words based on their lengths using a stable sorting algorithm. This ensures that words with the same length maintain their original order.
Step 3: Join the sorted words back into a single string, separated by spaces.
Step 4: Capitalize the first letter of the resulting string (the first word in the rearranged sentence).
Step 5: Return the rearranged sentence.

Key Insights

Insight 1: Splitting the sentence into words is crucial for individual manipulation.
Insight 2: Using a stable sorting algorithm (like Python's `sorted()` or implementing a stable sort in other languages) ensures the preservation of original word order when lengths are equal.
Insight 3: Correctly handling the capitalization of the first letter in the output sentence is a necessary detail.

Complexity Analysis

Time Complexity: O(n log n)

Space Complexity: O(n)

Topics

This problem involves: String, Sorting.

Companies

Asked at: Expedia.

Rearrange Words in a Sentence | LeetCode 1451

Medium

String Sorting

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

Solution Code

Available in:

import java.util.Arrays;
import java.util.Comparator;

class Solution {
    public String arrangeWords(String text) {
        String[] words = text.toLowerCase().split(" ");
        
        // Sort the words by their length. Java's Arrays.sort() with a Comparator achieves the same.
        Arrays.sort(words, Comparator.comparingInt(String::length));
        
        // Join the sorted words back into a sentence.
        String result_sentence = String.join(" ", words);
        
        // Capitalize the first letter of the new sentence.
        if (result_sentence.length() > 0) {
            result_sentence = result_sentence.substring(0, 1).toUpperCase() + result_sentence.substring(1);
        }
        
        return result_sentence;
    }
}

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Problem Info

Description

Problem 1451: Rearrange Words in a Sentence - Detailed explanation and solution approaches.

Topics

String Sorting

Companies

Expedia

Stats

Difficulty:Medium

Languages:4

Companies:1

Problem #:1451

Problem Explanation

💡

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

📋

Step-by-Step Solution

5 steps

1
Step 1: Convert the input sentence to lowercase and split it into an array (or vector) of individual words using spaces as delimiters.
2
Step 2: Sort the array of words based on their lengths using a stable sorting algorithm. This ensures that words with the same length maintain their original order.
3
Step 3: Join the sorted words back into a single string, separated by spaces.
4
Step 4: Capitalize the first letter of the resulting string (the first word in the rearranged sentence).
5
Step 5: Return the rearranged sentence.

💡

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 log n)

✨ Log-linear - Great for sorting algorithms!

💾

Space Complexity

O(n)

📦 Linear space - Memory grows with input size.

📝

Why?

The time complexity is dominated by the sorting algorithm used. Splitting and joining the string takes O(n) time, which is less than O(n log n). Space complexity is O(n) because we store the words in an array. In the C implementation, the space complexity might appear higher due to the multiple memory allocations using `malloc`, but asymptotically it is still O(n).

📖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

Counting Sort (if word length is bounded)

If the maximum length of a word is known and relatively small (e.g., less than 100), counting sort can be used to sort the words based on their lengths. This can achieve O(n + k) time complexity where n is the number of words and k is the maximum length of word

Time: O(n + k), where k is the maximum word lengthSpace: O(n + k)

Trade-offs: Counting sort is faster than O(n log n) algorithms like merge sort when the range of input values (word lengths) is small compared to the number of elements (words). However, it requires extra space for the counting array and is not efficient if the word length can be arbitrarily large.

✨

Key Insights

1Insight 1: Splitting the sentence into words is crucial for individual manipulation.
2Insight 2: Using a stable sorting algorithm (like Python's `sorted()` or implementing a stable sort in other languages) ensures the preservation of original word order when lengths are equal.
3Insight 3: Correctly handling the capitalization of the first letter in the output sentence is a necessary detail.

📚

Key Terms Explained

For Students

String

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

Sorting

Arranging elements in a specific order (ascending/descending).

Edge Cases

Edge case 1: Empty input sentence. The code should handle this gracefully, potentially returning an empty string or the original string (depending on the problem's specific requirements). In the given solutions, an empty sentence will result in correct capitalization handling because the if statement explicitly checks for result_sentence length.
Edge case 2: Sentence with only one word. The code should correctly capitalize the first letter and return the single-word sentence.
Edge case 3: Sentence with words of the same length. The stable sort ensures the original order is maintained among these words.

⚠️

Common Mistakes to Avoid

✗Common mistake 1: Using an unstable sorting algorithm. This can result in words of the same length being rearranged, violating the problem's requirements.

✗Common mistake 2: Incorrectly handling the capitalization of the first letter in the output sentence (e.g., not capitalizing it or capitalizing all letters).

✗Common mistake 3: Failing to deallocate dynamically allocated memory in C, leading to memory leaks.

✅

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 analysis, such as reordering words in a document based on their length for stylistic or readability purposes.

🏦Real-world scenario 2: Natural language processing, where sorting words based on length might be a preprocessing step in certain algorithms or models.

💡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

String problems often involve sliding windows or two pointers. Consider if the character set is limited (lowercase letters = only 26 possibilities).

🎯 Interview Tip

Consider multiple approaches before coding. Mentioning alternatives shows breadth of knowledge.

⚠️ Watch Out

Mutating strings directly in languages where strings are immutable. Use StringBuilder or arrays.

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 12, 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

The key insight here is recognizing the pattern. Once you see it, the solution becomes straightforward.

@AlgoMasterJul 13

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

@InterviewProJul 6

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

@CodeNinjaJun 29

❓

Frequently Asked Questions

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.

How much time should I spend on a medium problem in interviews?

In a 45-minute interview, aim for: 5-10 minutes understanding and clarifying, 5-10 minutes discussing approach, 20-25 minutes coding, and 5-10 minutes testing. If stuck after 15 minutes of thinking, verbalize your thought process and ask for hints.

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 "Rearrange Words in a Sentence"?

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

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