Given a string s, return a string containing the first vowel that occurs twice in the string. For this exercise, consider only the following vowels: a, e, i, o, u and their uppercase versions. If no vowel is doubled, return the empty string.

Complete the following file:


#include <cctype> #include <iostream> #include <string> using namespace std; /** Makes an lowercase version of a string. @param str a string @return a string with the characters in str converted to lowercase */ string lowercase(string str) { string result = str; // Make a copy of str for (int i = 0; i < result.length(); i++) { result[i] = tolower(result[i]); // Convert each character to lowercase } return result; } string firstDoubledVowel(string s) {