1133. Largest Unique Number

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.

Note:

1. 1 <= A.length <= 2000
2. 0 <= A[i] <= 1000