Given two lists of integers a and b, return the values that occur in a but not in b. Keep the order in which they first appear in a and list each value once.
Examples
Input: a = [3, 1, 3, 7, 1], b = [1, 9]
Output: [3, 7]
Constraints
0 <= len(a), len(b) <= 10**5- Target complexity: O(n + m) time.
Goals
- Compute a set difference while keeping list order
- Remove duplicates without sorting