Distinguishing extra element from two arrays?

Question

One of my friend was asked this question in an interview -

• You have given two integer arrays each of size 10.
• Both contains 9 equal elements (say 1 t

Answer 1

Two passes through the array will suffice.

1st pass: add every item in the shorter list to a hashmap (dict in python). 2nd pass: for each item in the longer list, check if key exist in hashmap (O(1) search time). If not, that key is the unique entry.

Total time complexity: O(2n) = O(n)