findNondup.js

// assumes the passed-in array contains a single non-duplicate. Finds the non-duplicate.
// O(3n) = O(n) runtime
findNonduplicate = (numbers) => {
  const count = {}

  // N
  numbers.forEach(n => count[n] = count[n] ? count[n] + 1 : 1)

  return (Object
    .keys(count)
    .map(n => [n, count[n]]) // N
    .find(([n, count]) => count === 1) // N
  )[0]

}

const numbers = [1, 2, 1, 2, 5, 4, 6, 3, 3, 4, 5]

console.log(findNonduplicate(numbers)) // 6