find_common_elements_in_list.py

# Implement your function below.
#Big O(n)
def common_elements(list1, list2):
    p1 = 0
    p2 = 0
    result = []
    while p1 < len(list1) and p2 < len(list2):
        if list1[p1] == list2[p2]:
            result.append(list1[p1])
            p1 += 1
            p2 += 1
        elif list1[p1] > list2[p2]:
            p2 += 1
        else:
            p1 += 1
    return result



# NOTE: The following input values will be used for testing your solution.
list_a1 = [1, 3, 4, 6, 7, 9]
list_a2 = [1, 2, 4, 5, 9, 10]
print(common_elements(list_a1,list_a2))

# common_elements(list_a1, list_a2) should return [1, 4, 9] (a list).

list_b1 = [1, 2, 9, 10, 11, 12]
list_b2 = [0, 1, 2, 3, 4, 5, 8, 9, 10, 12, 14, 15]
print(common_elements(list_b1,list_b2))
# common_elements(list_b1, list_b2) should return [1, 2, 9, 10, 12] (a list).

list_c1 = [0, 1, 2, 3, 4, 5]
list_c2 = [6, 7, 8, 9, 10, 11]
print(common_elements(list_c1,list_c2))
# common_elements(list_b1, list_b2) should return [] (an empty list).


OR

#Short one using comprehension
def common_elements(list1, list2):
    return [element for element in list1 if element in list2]

print(common_elements(list_a1, list_a2))