Count Integers That Appear Exactly Once

Given an array of integers, count how many distinct numbers occur exactly once.

Function Contract

Implement count_single_occurrences(numbers) -> int.

A number contributes one to the answer if its frequency in the entire array is exactly one.

Constraints and Clarifications

These numeric bounds are explicit practice assumptions.

• 0 <= len(numbers) <= 200000.
• Each value is an integer from -1000000000 through 1000000000.
• Repeated values do not contribute, regardless of where their occurrences appear.
• Return 0 for an empty array.
• Aim for expected O(n) time.

Examples

numbers = [4, 2, 1, 4, 4, 1, 3]
Output: 2

Only 2 and 3 appear once.

numbers = [5, 5, -1, -1]
Output: 0