What is the time complexity of Number of Valid Words in a Sentence?

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

What data structures are used to solve Number of Valid Words in a Sentence?

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

What are the key insights for solving Number of Valid Words in a Sentence?

The sentence needs to be split into individual words (tokens) before processing each word. A state machine or iterative approach can efficiently check the validity of each word based on the rules.

Which companies ask LeetCode 2047. Number of Valid Words in a Sentence in interviews?

This problem is frequently asked at Cisco.

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Number of Valid Words in a Sentence - LeetCode 2047 Solution

Number of Valid Words in a Sentence - Complete Solution Guide

Number of Valid Words in a Sentence is LeetCode problem 2047, a Easy level challenge. This complete guide provides step-by-step explanations, multiple solution approaches, and optimized code in python3, java, cpp, c.

Problem Statement

A sentence consists of lowercase letters ( 'a' to 'z' ), digits ( '0' to '9' ), hyphens ( '-' ), punctuation marks ( '!' , '.' , and ',' ), and spaces ( ' ' ) only. Each sentence can be broken down into one or more tokens separated by one or more spaces ' ' . A token is a valid word if all three of the following are true: It only contains lowercase letters, hyphens, and/or punctuation ( no digits). There is at most one hyphen '-' . If present, it must be surrounded by lowercase characters ( "a-b

Detailed Explanation

The problem asks you to count the number of valid words in a given sentence. A valid word is defined by three rules: 1. **No digits:** The word must contain only lowercase letters, hyphens, and punctuation marks (!, ., ,). 2. **At most one hyphen:** If a hyphen exists, it must be surrounded by lowercase letters (e.g., "a-b" is valid, but "-ab" and "ab-" are not). 3. **At most one punctuation mark:** If a punctuation mark is present, it must be at the end of the word (e.g., "ab," is valid, but "a!b" is not). The input is a string representing the sentence, and the output is an integer representing the number of valid words.

Solution Approach

The provided solutions use a common approach. They first split the input sentence into tokens using whitespace as the delimiter. Then, for each token, they iterate through its characters, checking for violations of the three rules. If a violation is found, the word is deemed invalid. A counter tracks the number of valid words, which is returned as the final result. The `isValid` helper function (in Java and C++) encapsulates the word validation logic for better code organization and readability.

Step-by-Step Algorithm

Split the input sentence into tokens using whitespace as the delimiter.
Iterate through each token:
- Initialize counters for hyphens and punctuation marks.
- Iterate through the characters of the current token:
- Check for digits; if found, mark the token as invalid.
- Check for hyphens; if found, increment the hyphen counter and check for rule violations (multiple hyphens, hyphen at the beginning or end, or hyphen not surrounded by lowercase letters).
- Check for punctuation marks; if found, increment the punctuation counter and check for rule violations (multiple punctuation marks, punctuation mark not at the end).
- If any rule violation is detected, break the inner loop and mark the token as invalid.
- If no rule violation is detected, increment the count of valid words.
Return the total count of valid words.

Key Insights

The sentence needs to be split into individual words (tokens) before processing each word.
A state machine or iterative approach can efficiently check the validity of each word based on the rules.
Proper handling of edge cases like empty tokens, words with multiple hyphens or punctuation, and words starting or ending with hyphens is crucial.

Complexity Analysis

Time Complexity: O(N)

Space Complexity: O(N)

Topics

This problem involves: String.

Companies

Asked at: Cisco.

Number of Valid Words in a Sentence | LeetCode 2047

Easy

String

Time: O(N)Space: O(N)

Solution Code

Available in:

class Solution {
    public int countValidWords(String sentence) {
        int count = 0;
        String[] tokens = sentence.split("\\s+");
        for (String token : tokens) {
            if (isValid(token)) {
                count++;
            }
        }
        return count;
    }

    private boolean isValid(String token) {
        if (token.isEmpty()) {
            return false;
        }

        int hyphenCount = 0;
        int punctuationCount = 0;

        for (int i = 0; i < token.length(); i++) {
            char c = token.charAt(i);

            if (Character.isDigit(c)) {
                return false;
            } else if (c == '-') {
                hyphenCount++;
                if (hyphenCount > 1) {
                    return false;
                }
                if (i == 0 || i == token.length() - 1) {
                    return false;
                }
                if (!Character.isLowerCase(token.charAt(i - 1)) || !Character.isLowerCase(token.charAt(i + 1))) {
                    return false;
                }
            } else if (c == '!' || c == '.' || c == ',') {
                punctuationCount++;
                if (punctuationCount > 1) {
                    return false;
                }
                if (i != token.length() - 1) {
                    return false;
                }
            } else if (!Character.isLowerCase(c)) {
                return false;
            }
        }

        return true;
    }
}

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Problem Info

Description

Problem 2047: Number of Valid Words in a Sentence - Detailed explanation and solution approaches.

Topics

String

Companies

Cisco

Stats

Difficulty:Easy

Languages:4

Companies:1

Problem #:2047

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

10 steps

1
Split the input sentence into tokens using whitespace as the delimiter.
2
Iterate through each token:
3
- Initialize counters for hyphens and punctuation marks.
4
- Iterate through the characters of the current token:
5
- Check for digits; if found, mark the token as invalid.
6
- Check for hyphens; if found, increment the hyphen counter and check for rule violations (multiple hyphens, hyphen at the beginning or end, or hyphen not surrounded by lowercase letters).
7
- Check for punctuation marks; if found, increment the punctuation counter and check for rule violations (multiple punctuation marks, punctuation mark not at the end).
8
- If any rule violation is detected, break the inner loop and mark the token as invalid.
9
- If no rule violation is detected, increment the count of valid words.
10
Return the total count of valid words.

💡

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)

💾

Space Complexity

O(N)

📝

Why?

The dominant factor in both time and space complexity is the size of the input sentence. The algorithm processes each character once in the worst case. The memory used is proportional to the length of the sentence.

📖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

Regular Expression

A regular expression can be used to match valid words according to the specified rules. This approach can be more concise but might be less readable for beginners.

Time: O(N)Space: O(1)

Trade-offs: Regular expressions offer conciseness but may be less efficient than iterative approaches in some cases and can be harder to debug and maintain if complex.

✨

Key Insights

1The sentence needs to be split into individual words (tokens) before processing each word.
2A state machine or iterative approach can efficiently check the validity of each word based on the rules.
3Proper handling of edge cases like empty tokens, words with multiple hyphens or punctuation, and words starting or ending with hyphens is crucial.

📚

Key Terms Explained

For Students

String

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

Edge Cases

Empty sentence: The code handles this by checking for empty tokens after splitting the sentence. The counter only increments if a token is not empty.
Sentence with only spaces: The code correctly handles this; the `split` function will return an empty array, and the loop will not execute, resulting in a valid count of 0.
Words with only hyphens or punctuation: The code correctly identifies such words as invalid.

⚠️

Common Mistakes to Avoid

✗Forgetting to handle empty tokens after splitting the sentence.

✗Incorrectly checking the hyphen's position or the punctuation mark's position.

✗Not handling multiple hyphens or punctuation marks correctly.

✗Failing to ensure that characters surrounding a hyphen are lowercase letters.

✅

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?

💼Text preprocessing in natural language processing (NLP): Cleaning and validating input text before further processing.

🏦Data validation in forms or other input fields: Ensuring that user input conforms to specific rules.

🎮Simple lexical analysis: Identifying and classifying tokens in a program's source code.

💡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

Use this problem to demonstrate clean code practices. Variable naming and code organization matter.

⚠️ 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: August 7, 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 14

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

@CodeNinjaJul 7

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

@TechLeadJun 30

❓

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.

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 "Number of Valid Words in a Sentence"?

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

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