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**/Constraints

The challenge in this problem is in meeting its (arbitrary) constraints:

Do not convert into strings or manipulate strings at all.
Do not create any other data structures.
In particular, don't instantiate a new array.
The big-O of the solution should be O(n). **/

let vowelDoubler = (letters) => {
  let wordLength = letters.length;
  let vowels = ['a', 'e', 'i', 'o', 'u'];
  let numVowel = 0;
  //count vowels
  for (let i=0; i < wordLength; i++) {
    if(vowels.includes(letters[i])) {
      numVowel++
    }
  }
  let originalLength = letters.length - 1;
  wordLength += numVowel - 1;
  for(let i=wordLength; i > 0; i--) {
    letters[i] = letters[originalLength];
    if(vowels.includes(letters[i])) {
      i--;
      letters[i] = letters[originalLength];
    }
    originalLength--;
  }
  return letters
}