Only in the First

You get two ArrayLists of integers, a and b.

Return a new ArrayList with the values of a that are not in b.

Each value goes into the result only once. Keep the values in the order they first appear in a.

If every value of a is also in b, return an empty ArrayList.

Example 1:

Input: a = [1, 2, 3, 4], b = [2, 4]
Output: [1, 3]
Explanation: 2 and 4 are in b, so they are left out.

Example 2:

Input: a = [5, 6, 5, 7], b = [7]
Output: [5, 6]
Explanation: 7 is in b.
5 appears twice in a, but only once in the result.

Example 3:

Input: a = [1, 2], b = [2, 3, 1]
Output: []
Explanation: Both values of a are also in b, so nothing is left.

Constraints:

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