cpp
class Solution {
public:
    int vowelStrings(vector<string>& words, int left, int right) {
        int ans = 0;
        set<char> s{'a','e','i','o','u'};
        for(;left<=right;left++){
            if(s.count(words[left][0])&&s.count(words[left][words[left].size()-1])){
                ans++;
            }
        }
        return ans;
    }
};


// 
class Solution {
public:
    int vowelStrings(const vector<string>& words, int left, int right) {
        int ans = 0;
        set<char> s{'a','e','i','o','u'};
        int n = words.size();
        vector<int> prefixSum(n + 1, 0);
        
        for (int i = 1; i <= n; i++) {
            prefixSum[i] = prefixSum[i - 1] + (s.count(words[i - 1][0]) && s.count(words[i - 1].back()));
        }
        
        return prefixSum[right + 1] - prefixSum[left];
    }
};