#1456

Medium

auto_awesomeSliding window with running state updates

LeetCode Problem Workspace

Maximum Number of Vowels in a Substring of Given Length

Find the maximum number of vowels in a substring of a given length in a string using a sliding window approach.

Problem Statement

You are given a string s and an integer k. Your task is to return the maximum number of vowels in any substring of length k within s.

Vowel letters are 'a', 'e', 'i', 'o', and 'u'. Implement a solution that efficiently handles the problem by keeping track of the vowels in the current window of length k.

Examples

Example 1

Input: s = "abciiidef", k = 3

Output: 3

The substring "iii" contains 3 vowel letters.

Example 2

Input: s = "aeiou", k = 2

Output: 2

Any substring of length 2 contains 2 vowels.

Example 3

Input: s = "leetcode", k = 3

Output: 2

"lee", "eet" and "ode" contain 2 vowels.

Constraints

1 <= s.length <= 105
s consists of lowercase English letters.
1 <= k <= s.length

Solution Approach

Sliding Window Approach

Initialize a sliding window of size k and count the vowels in the initial window. As you slide the window, subtract the impact of the character that is sliding out of the window and add the impact of the character that is entering the window. Keep track of the maximum vowel count throughout this process.

Efficient Vowel Checking

While iterating through the string, check each character to see if it's a vowel. This can be done by checking against a predefined set of vowels, optimizing the time complexity. This approach ensures that the vowel count is updated efficiently as the window slides.

Edge Case Handling

Consider edge cases such as when k equals the length of the string, when there are no vowels, or when all characters are vowels. Ensure the solution works for these cases without additional complexity.

Complexity Analysis

Metric	Value
Time	Depends on the final approach
Space	Depends on the final approach

The time complexity of this approach is O(n), where n is the length of the string, as we traverse the string once and update the vowel count for each sliding window operation. The space complexity is O(1), as we only maintain a count of vowels in the current window and a few auxiliary variables.

What Interviewers Usually Probe

The candidate demonstrates a strong understanding of the sliding window technique.
The candidate optimizes the solution with a constant space complexity.
The candidate handles edge cases effectively.

Common Pitfalls or Variants

Common pitfalls

Overcomplicating the problem by trying to find a solution without using a sliding window.
Failing to maintain the correct vowel count during the sliding window update step.
Not considering edge cases such as no vowels or all vowels in the string.

Follow-up variants

What if the window size k can change dynamically?
Can the problem be solved with a brute force approach, and if so, how does it compare to the sliding window method?
How would this solution change if we needed to track the substring with the maximum vowel count, not just the count itself?

FAQ

What is the best approach for solving the 'Maximum Number of Vowels in a Substring of Given Length' problem?

The best approach is to use the sliding window technique, maintaining a count of vowels in the current window of size k.

How does the sliding window technique work in this problem?

The sliding window technique works by maintaining a window of size k, adjusting the vowel count as characters enter and exit the window.

What are the time and space complexities of this approach?

The time complexity is O(n) and the space complexity is O(1), as we only maintain a count of vowels in the current window.

What are common mistakes to avoid in this problem?

Common mistakes include failing to update the vowel count correctly when sliding the window and not considering edge cases like no vowels or all vowels.

How does GhostInterview help with this problem?

GhostInterview helps by providing hints, ensuring you understand the sliding window technique, and guiding you through handling edge cases.

terminal

Solution

Solution 1: Sliding Window

First, we count the number of vowels in the first $k$ characters, denoted as $cnt$, and initialize the answer $ans$ as $cnt$.

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class Solution:
    def maxVowels(self, s: str, k: int) -> int:
        vowels = set("aeiou")
        ans = cnt = sum(c in vowels for c in s[:k])
        for i in range(k, len(s)):
            cnt += int(s[i] in vowels) - int(s[i - k] in vowels)
            ans = max(ans, cnt)
        return ans

Continue Practicing

#1358 number of substrings containing all three characters #1297 maximum number of occurrences of a substring #1234 replace the substring for balanced string