title: Find the element that appears once
body: <p>Find the element that appears once</p>
  
  <p>Given an array where every element occurs three times, except one element which occurs only once. Find the element that occurs once. </p>
  
  <p>Expected time complexity is O(n) and O(1) extra space.</p>
  
  <p>Examples: </p>
  
  <p>Input: arr[] = {12, 1, 12, 3, 12, 1, 1, 2, 3, 3}</p>
  
  <p>Output: 2</p>