title: O(n) algorithm, find the missing integer
body: <p>An array A contains n-1 unique integers in the range [0, n-1], that is, there is one number from this range that is not in A. Design an O(n) algorithm for finding that number. Allowed to use only O(1) additional space besides the array A itself.</p>
  
  <p>need some help for this question,</p>