Problem URL: Largest unique number

We will use a hashmap to store the frequency of each number. Then we will iterate over the hashmap and return the maximum number whose frequency is 1. If there is no such number, we will return -1.

class Solution:
    def largestUniqueNumber(self, nums: List[int]) -> int:
        freq = defaultdict(int)
        for num in nums:
            freq[num] += 1

        res = -1
        for num in freq:
            if freq[num] == 1:
                res = max(res, num)

        return res

Time complexity: O(n)
Space complexity: O(n)

This can be done in one line using Counter from collections module with just one line.

class Solution:
    def largestUniqueNumber(self, nums: List[int]) -> int:
        return max((v for v, c in collections.Counter(nums).items() if c == 1), default=-1)