c - Finding common elements in two arrays of different size

Question

I have a problem to find common elements in two arrays and that's of different size.

Take , Array A1 of size n and Array A2 of size m, and m != n

So far, I've tried to iterate lists one by one and copy elements to another list. If the element already contains mark it, but I know it's not a good solution.

Answer 1

Sort the arrays. Then iterate through them with two pointers, always advancing the one pointing to the smaller value. When they point to equal values, you have a common value. This will be O(n log n+m log m) where n and m are the sizes of the two lists. It's just like a merge in merge sort, but where you only produce output when the values being pointed to are equal.

def common_elements(a, b):
  a.sort()
  b.sort()
  i, j = 0, 0
  common = []
  while i < len(a) and j < len(b):
    if a[i] == b[j]:
      common.append(a[i])
      i += 1
      j += 1
    elif a[i] < b[j]:
      i += 1
    else:
      j += 1
  return common

print 'Common values:', ', '.join(map(str, common_elements([1, 2, 4, 8], [1, 4, 9])))

outputs

Common values: 1, 4

If the elements aren't comparable, throw the elements from one list into a hashmap and check the elements in the second list against the hashmap.