What is the time complexity of Count Vowel Substrings of a String?

The optimal solution has a time complexity of O(n^3).

What data structures are used to solve Count Vowel Substrings of a String?

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

What are the key insights for solving Count Vowel Substrings of a String?

Insight 1: The problem requires checking all possible substrings, necessitating nested loops to iterate through all starting and ending positions. Insight 2: Using a set to efficiently track the unique vowels present in each substring avoids redundant checks. Alternatively, boolean flags can be used but are slightly less efficient.

Which companies ask LeetCode 2062. Count Vowel Substrings of a String in interviews?

This problem is frequently asked at BNY Mellon, Commvault, PayPal, Snowflake.

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Count Vowel Substrings of a String - LeetCode 2062 Solution

Count Vowel Substrings of a String - Complete Solution Guide

Count Vowel Substrings of a String is LeetCode problem 2062, 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 substring is a contiguous (non-empty) sequence of characters within a string. A vowel substring is a substring that only consists of vowels ( 'a' , 'e' , 'i' , 'o' , and 'u' ) and has all five vowels present in it. Given a string word , return the number of vowel substrings in word . Example 1: Input: word = "aeiouu" Output: 2 Explanation: The vowel substrings of word are as follows (underlined): - " aeiou u" - " aeiouu " Example 2: Input: word = "unicornarihan" Output: 0 Explanation: Not all

Detailed Explanation

The problem asks to count the number of substrings within a given string that meet two conditions: 1) the substring contains only vowels (a, e, i, o, u), and 2) the substring contains all five vowels. The input is a string `word`, and the output is an integer representing the count of such vowel substrings. Constraints limit the input string's length to a maximum of 100 characters and ensure that the string contains only lowercase English letters.

Solution Approach

The provided solutions all employ a brute-force approach. They iterate through all possible substrings of the input string. For each substring, they check if it consists only of vowels and contains all five vowels. If both conditions are true, the counter is incremented. This approach systematically examines every possible substring, ensuring that no qualifying substring is missed.

Step-by-Step Algorithm

Step 1: Iterate through all possible starting indices `i` of substrings.
Step 2: For each starting index `i`, iterate through all possible ending indices `j` (j >= i).
Step 3: Extract the substring `word[i:j+1]`.
Step 4: Check if the substring contains only vowels. This can be done by iterating through the substring's characters and checking if each is in the set {'a', 'e', 'i', 'o', 'u'}.
Step 5: If the substring contains only vowels, check if it contains all five vowels using a set or boolean flags. A set is more efficient.
Step 6: If both conditions are met, increment the counter.
Step 7: After iterating through all substrings, return the counter.

Key Insights

Insight 1: The problem requires checking all possible substrings, necessitating nested loops to iterate through all starting and ending positions.
Insight 2: Using a set to efficiently track the unique vowels present in each substring avoids redundant checks. Alternatively, boolean flags can be used but are slightly less efficient.
Insight 3: Optimizations are possible, but the brute-force approach is sufficiently efficient for the given constraints (string length <= 100). More advanced techniques may be necessary for significantly larger inputs.

Complexity Analysis

Time Complexity: O(n^3)

Space Complexity: O(1)

Topics

This problem involves: Hash Table, String.

Companies

Asked at: BNY Mellon, Commvault, PayPal, Snowflake.

Count Vowel Substrings of a String | LeetCode 2062

Easy

Hash Table String

Time: O(n^3)Space: O(1)

Solution Code

Available in:

class Solution {
    public int countVowelSubstrings(String word) {
        int count = 0;
        for (int i = 0; i < word.length(); i++) {
            for (int j = i; j < word.length(); j++) {
                String sub = word.substring(i, j + 1);
                if (isVowelSubstring(sub)) {
                    count++;
                }
            }
        }
        return count;
    }

    private boolean isVowelSubstring(String s) {
        String vowels = "aeiou";
        boolean a = false, e = false, i = false, o = false, u = false;
        for (int k = 0; k < s.length(); k++) {
            char c = s.charAt(k);
            if (vowels.indexOf(c) == -1) {
                return false;
            }
            if (c == 'a') a = true;
            if (c == 'e') e = true;
            if (c == 'i') i = true;
            if (c == 'o') o = true;
            if (c == 'u') u = true;
        }
        return a && e && i && o && u;
    }
}

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Problem Info

Description

Problem 2062: Count Vowel Substrings of a String - Detailed explanation and solution approaches.

Topics

Hash Table String

Companies

BNY MellonCommvaultPayPalSnowflake

Stats

Difficulty:Easy

Languages:4

Companies:4

Problem #:2062

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

7 steps

1
Step 1: Iterate through all possible starting indices `i` of substrings.
2
Step 2: For each starting index `i`, iterate through all possible ending indices `j` (j >= i).
3
Step 3: Extract the substring `word[i:j+1]`.
4
Step 4: Check if the substring contains only vowels. This can be done by iterating through the substring's characters and checking if each is in the set {'a', 'e', 'i', 'o', 'u'}.
5
Step 5: If the substring contains only vowels, check if it contains all five vowels using a set or boolean flags. A set is more efficient.
6
Step 6: If both conditions are met, increment the counter.
7
Step 7: After iterating through all substrings, return the counter.

💡

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^3)

💾

Space Complexity

O(1)

💪 Constant space - Uses same memory regardless of input!

📝

Why?

The nested loops iterate through O(n^2) substrings. For each substring, checking if it contains only vowels and all five vowels takes O(n) time in the worst case (iterating through the substring). This results in O(n^3) overall time complexity. The space complexity is O(1) because the solutions only use a constant amount of extra space regardless of input size. The set (or boolean flags) used to track vowels has a constant 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

Optimized Brute-Force

Instead of checking all substrings, start with a substring containing at least one vowel. Expand the substring to the right until a non-vowel is encountered or all five vowels are found.

Time: O(n^2)Space: O(1)

Trade-offs: This approach avoids unnecessary checks for substrings that obviously don't meet the conditions. It's faster but still O(n^2) in worst case (when many substrings contain only vowels but are missing one or more vowels).

✨

Key Insights

1Insight 1: The problem requires checking all possible substrings, necessitating nested loops to iterate through all starting and ending positions.
2Insight 2: Using a set to efficiently track the unique vowels present in each substring avoids redundant checks. Alternatively, boolean flags can be used but are slightly less efficient.
3Insight 3: Optimizations are possible, but the brute-force approach is sufficiently efficient for the given constraints (string length <= 100). More advanced techniques may be necessary for significantly larger inputs.

📚

Key Terms Explained

For Students

Hash Table

A data structure that maps keys to values for quick lookup. Like a dictionary!

String

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

Edge Cases

Edge case 1: Empty string (""). The solutions correctly handle this case by not entering the nested loops, resulting in a count of 0.
Edge case 2: String with no vowels. The solutions correctly identify that there are no vowel substrings and return 0.

⚠️

Common Mistakes to Avoid

✗Common mistake 1: Forgetting to check for all five vowels. Many beginners might only verify that the substring contains only vowels, neglecting the crucial requirement of having all five.

✗Common mistake 2: Inefficiently checking for vowel presence. Using a linear search within the substring for each vowel is less efficient than using a set or boolean flags.

✅

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 analysis and natural language processing. This type of substring analysis can be used for tasks such as identifying specific vowel patterns in words or text segments.

🏦Real-world scenario 2: Bioinformatics. Similar substring pattern matching is applied to DNA or RNA sequences to identify specific genetic markers or motifs.

💡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

Hash tables trade space for time. Before using one, consider if the problem constraints allow the extra memory. In interviews, always mention this trade-off.

🎯 Interview Tip

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

⚠️ Watch Out

Don't forget to handle key collisions and null/undefined keys. Consider what happens with duplicate values.

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 22, 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

❓

Frequently Asked Questions

What are the trade-offs of using a hash table?

Hash tables provide O(1) average-case lookup, insertion, and deletion, making them ideal for frequency counting and duplicate detection. However, they use O(n) extra space and may degrade to O(n) time in worst-case scenarios with hash collisions. Always consider if the space trade-off is acceptable.

How do I handle string manipulation efficiently?

In languages with immutable strings (Java, Python, C#), use StringBuilder/list to avoid O(n) concatenation cost. Consider using character arrays for in-place modifications. For pattern matching, consider KMP or Rabin-Karp for O(n+m) complexity.

How should I approach easy-level algorithm problems?

Easy problems build foundational skills. Focus on: 1) Understanding the problem completely before coding, 2) Writing clean, readable code, 3) Handling all edge cases (empty input, single element, etc.), 4) Analyzing time and space complexity. Don't skip complexity analysis even for simple solutions.

How should I practice algorithm problems effectively?

Focus on understanding patterns rather than memorizing solutions. After solving a problem, review optimal solutions and understand the intuition. Group similar problems to recognize patterns. Practice explaining your approach out loud. Review problems after a few days to test retention.

What is the optimal approach for "Count Vowel Substrings of a String"?

For this easy-level Hash Table 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.

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