Find_The_Duplicate_Number.js

/*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.
*/



function findDup( arr ){
    return arr.filter(x => arr.indexOf(x) != arr.lastIndexOf(x))[0]
  }