Common Values in Python

Given two lists of integers, a and b, return how many different values appear in both of them.

A value counts only once, however many times it appears in a or in b.

Example 1:

Input: a = [1, 2, 3, 4], b = [3, 4, 5]
Output: 2
Explanation: 3 and 4 are in both. 1 and 2 are only in a, and 5 is only in b.

Example 2:

Input: a = [7, 7, 8], b = [7, 9, 7]
Output: 1
Explanation: 7 is the only value in both. It appears twice in each, but it counts once.

Example 3:

Input: a = [1, 2], b = [3, 4]
Output: 0
Explanation: No value is in both.

Constraints:

• a and b each have at least one number.
• Every number is between -10⁹ and 10⁹.