/**
 * @param {string} string
 */
function solution (string) {
  const lettersCount = {}
  for (let index = 0; index < string.length; index++) {
    const character = string[index]
    if (lettersCount[character] == null) {
      lettersCount[character] = {
        total: 1,
        firstIndex: index,
        value: character
      }
    } else {
      lettersCount[character].total += 1
    }
  }

  let result = null
  for (const character in lettersCount) {
    const characterObject = lettersCount[character]
    if (characterObject.total === 1) {
      if (result == null) {
        result = characterObject
      } else if (characterObject.firstIndex < result.firstIndex) {
        result = characterObject
      }
    }
  }

  if (result == null) {
    return ''
  }
  return result.value
}

module.exports = solution