Badges are numbered 1 to n. The list seen records the badges that were scanned today (a badge may be scanned several times). Return a sorted list of badge numbers that were never scanned.
Examples
Input: n = 6, seen = [2, 5, 2, 1]
Output: [3, 4, 6]
Input: n = 3, seen = [1, 2, 3, 3]
Output: []
Constraints
0 <= n <= 10**5,1 <= seen[i] <= n- Target complexity: O(n + len(seen)) time.
Goals
- Build a set for O(1) membership
- Produce a sorted result by iterating a range, not by sorting