Unique Values Across Two Arrays II
Given two arrays, return only the values that occur exactly once across both, in first-seen order: appearOnce(a, b).
appearOnce([1, 2, 2, 3], [3, 4]) // [1, 4]
Values are numbers and strings, and 1 and "1" are different values. Repeats within one array count too.
Hints
Hint 1
Count every value in a Map over both arrays, then keep the values whose count is 1. A Map keeps first-seen order.