Missing Numbers

Problem

Given an array nums of n integers where nums[i] is in the range [1, n], return an array of all the integers in the range [1, n] that do not appear in nums.

Examples

findDisappearedNumbers([3,4,5,2,8,2,3,1]) 
// returns [6,7]
// 6 and 7 are the numbers in the
// range 1 to 8 that are not in the array.

findDisappearedNumbers([2,2]) 
// returns [1]
// 1 is the number in the
// range 1 to 2 that is not in the array.