title: finding missing name from two lists
body: <p>This is an example to make the situation clear. 
  Let's say we have two lists contain some names. I call them original (O) and modified (M) lists. Note, M has missed names. I don't have access to O, but I have access to M. What I have from O is a list , I called it O1, that follows two conditions: (1) in the case of repetition, only the most recent name should be considered, (2) the order of O1 is LIFO of O. </p>
  
  <p>As an example, Imagine the original list contains O={n1,n2,n3,n2,n1,n3,n1,n3} where n1 is the first person who wrote his name in the first, fifth, and seventh position. So, what I have is O1={n3,n1,n2}, those two conditions have applied. Now, the modified list contains M={n2,n3,n2,n1,n3,n1} ( two names have been removed(n1, n3)), and by following those two conditions I can create M1={n1,n3,n2}.  By comparing O1, and M1</p>
  
  <p>O1={n3,n1,n2}</p>
  
  <p>M1={n1,n3,n2}</p>
  
  <p>I can find n3 as a missing name because the position of n3 has been modified in my generated list (M1). </p>
  
  <p>How I can catch n1 as a missing name?</p>