2559. Count Vowel Strings in Ranges

🟧 Medium

You are given a 0-indexed array of strings words and a 2D array of integers queries.

Each query queries[i] = [li, ri] asks us to find the number of strings present in the range li to ri (both inclusive) of words that start and end with a vowel.

Return an array ans of size queries.length, where ans[i] is the answer to the i^th query.

Note that the vowel letters are 'a', 'e', 'i', 'o', and 'u'.

Example 1

Input: words = ["aba","bcb","ece","aa","e"], queries = [[0,2],[1,4],[1,1]] Output: [2,3,0] Explanation: The strings starting and ending with a vowel are "aba", "ece", "aa" and "e". The answer to the query [0,2] is 2 (strings "aba" and "ece"). to query [1,4] is 3 (strings "ece", "aa", "e"). to query [1,1] is 0. We return [2,3,0].

Example 2

Input: words = ["a","e","i"], queries = [[0,2],[0,1],[2,2]] Output: [3,2,1] Explanation: Every string satisfies the conditions, so we return [3,2,1].

Constraints

• 1 <= words.length <= 10^5

• 1 <= words[i].length <= 40

• words[i] consists only of lowercase English letters.

• sum(words[i].length) <= 3 * 10^5

• 1 <= queries.length <= 10^5

• 0 <= li <= ri < words.length

Hint 1

Precompute the prefix sum of strings that start and end with vowels.

Hint 2

Use unordered_set to store vowels.

Hint 3

Check if the first and last characters of the string are present in the vowels set.

Hint 4

Subtract prefix sum for range [l-1, r] to find the number of strings starting and ending with vowels.

Solution

My Solution

func vowelStrings(words []string, queries [][]int) []int {
    vowels := map[byte]bool{
        'a':true,
        'e':true,
        'i':true,
        'o':true,
        'u':true,
    }
    ans := make([]int, len(queries))
    vowelCounts := make([]int, len(words)+1)

    for i, word := range words {
        if vowels[word[0]] && vowels[word[len(word)-1]] {
            vowelCounts[i+1]++
        }
        vowelCounts[i+1]+=vowelCounts[i]
    }

    for i, query := range queries {
        ans[i] = vowelCounts[query[1]+1]-vowelCounts[query[0]]
    }

    return ans
}

result

Leetcode: link