title: Sum of MEX of all subarrays of the given array
body: <blockquote>
  <p>Find the sum of MEX of all subarrays of the given array. The MEX
  (minimum excluded) of an array is the smallest non-negative integer
  that does not belong to the array.</p>
  <p>For instance: The MEX of [2,2,1] is
  0, because 0 does not belong to the array. The MEX of [3,1,0,1] is 2,
  because 0 and 1 belong to the array, but 2 does not. The MEX of
  [0,3,1,2] is 4 because 0, 1, 2 and 3 belong to the array, but 4 does
  not. It's worth mentioning that the MEX of an array of length n is
  always between 0 and n inclusive.</p>
  <p>Constraints:
  n&lt;=10^5,a[i]&lt;=n</p>
  </blockquote>
  <blockquote>
  <p>I thought of a O(n^2) solutions which will not pass the given constraints. We need to find a O(nlogn) or O(n) solution</p>
  </blockquote>