title: Finding duplicates in O(n) time and O(1) space
body: <p>Input: Given an array of n elements which contains elements from 0 to n-1, with any of these numbers appearing any number of times. </p>
  
  <p>Goal : To find these repeating numbers in O(n) and using only constant memory space.</p>
  
  <p>For example, let n be 7 and array be {1, 2, 3, 1, 3, 0, 6}, the answer should be 1 &amp; 3.
  I checked similar questions here but the answers used some data structures like <code>HashSet</code> etc. </p>
  
  <p>Any efficient algorithm for the same? </p>