Question

Earlier today I asked similar problem to find the maximum element which is common in two arrays. I got a couple of good solutions here ( Find the maximum element which is common in two arrays?).

Now it occurred to me, what if instead of two arrays we have to find the maximum element which is common in n different arrays?

Example:

 array1 = [1,5,2,4,6,88,34]
 array2 = [1,5,6,2,34]
 array3 = [1,34]
 array4 = [7,99,34]

Here the maximum element which is common in all the arrays is 34.

Is it a good idea to create a hashmap of the array1, array2 ….. array(N-1) separately and then check every element of arrayN in each of these hashmaps keeping track of maximum element (when present in all the hashmaps)?

Can we have better solutions than this?

Answer 1

For each array A_n:
  Add all elements in A_n to a hashset, H_n

Create a hashmap, M, which maps values to counts.
For each hashset H_n:
  For each value, v, in H_n:
    M[v]++

Go through M for the highest value with count == N

This will run in O(n) time and space, where n is the total number of elements in all arrays. It also properly deals with elements being duplicated in a single array, which you didn’t mention but which might cause problems for some algorithms. If you know that you won’t have duplicate elements in a single array you can skip the first step and add values to M directly from the arrays.