What is the Count Vowels in Ranges pattern?

The Count Vowels in Ranges is a common coding interview pattern in Prefix Sum problems. It provides a structured approach to solving related algorithm challenges efficiently.

When should I use Count Vowels in Ranges?

Use the Count Vowels in Ranges pattern when you encounter problems matching its characteristics. Check the "When to Use" and "Clues" sections in the lesson for specific indicators.

Is Count Vowels in Ranges asked in FAANG interviews?

Yes, Count Vowels in Ranges is an essential pattern commonly tested in FAANG and top tech company technical interviews.

Prefix Sum

Medium

Count Vowels in Ranges

Efficiently answer multiple queries about the number of vowels in a range.

10 min read

Solve on LeetCode

Problem Understanding

You are given a string array words and a 2D array queries. Each query [l, r] asks for the number of strings in words[l...r] that start and end with a vowel.

We need to answer many queries efficiently.

Strategy

Preprocessing: Create a boolean/integer array is_valid where is_valid[i] = 1 if words[i] starts/ends with vowel, else 0.
Prefix Sum: Compute prefix sum P of is_valid. P[i] = count of valid words in words[0...i-1].
Query: Answer for [l, r] is P[r+1] - P[l].

Vowels: a, e, i, o, u

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