find-the-duplicated-number-in-a-consecutive-unsorted-list.md

Find The Duplicated Number in a Consecutive Unsorted List ^{7 Kyu}

^{LINK TO THE KATA - ARRAYS ALGORITHMS}

Description

You are given an array of n+1 integers 1 through n. In addition there is a single duplicate integer.

The array is unsorted.

An example valid array would be [3, 2, 5, 1, 3, 4]. It has the integers 1 through 5 and 3 is duplicated. [1, 2, 4, 5, 5] would not be valid as it is missing 3.

You should return the duplicate value as a single integer.

Solution

const findDup = array => {
  return array.find(
    number => array.indexOf(number) !== array.lastIndexOf(number),
  )
}