Given an array of integers A, return the largest integer that only occurs once.

largest_integer.py

"""
Given an array of integers A, return the largest integer that only occurs once.

If no integer occurs once, return -1.

Example 1:

Input: [5,7,3,9,4,9,8,3,1]
Output: 8
Explanation: 
The maximum integer in the array is 9 but it is repeated. The number 8 occurs only once, so it's the answer.
Example 2:

Input: [9,9,8,8]
Output: -1
Explanation: 
There is no number that occurs only once.
"""
class Solution:
    def largestUniqueNumber(self, A: List[int]) -> int:
        if not A:
            return -1
        numbers = {}
        for number in A:
            if number in numbers:
                numbers[number] = numbers[number] + 1
            else:
                numbers[number] = 1
        largest = -1
        for number in numbers:
            if numbers[number] > 1:
                continue
            if largest < number:
                largest = number
        return largest