title: Given an array that contains all elements thrice except one. Find the element which occurs once.
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/5208343/find-a-special-number-in-an-array">Find a special number in an array</a>  </p>
  </blockquote>
  
  
  
  <p>Given an array that contains all elements thrice except one. Find the element which occurs once. Time complexity- O(n). Space complexity- O(1).</p>
  
  <p>Eg. ar[10]={1,2,1,2,5,5,5,7,1,2}.
      ans= 7.</p>