Count Vowels in Ranges

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

10 min read

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

Interactive Visualization

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See how we can answer multiple range queries in O(1) time after O(N) preprocessing.

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