LeetCode 3843: First Element with Unique Frequency

Meta Data

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Difficulty: medium First Attempt: 2026-02-16

• Submissions: Accepted (first try)

Approach

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Two levels of counting (Hash Table Template 1):

• cnt: value -> count.
• freq_of_freq: count -> how many distinct values have that count.

“Unique count” means freq_of_freq[cnt[x]] == 1. For the first one, scan the original array from the left.

My Solution

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class Solution:
    def firstUniqueFreq(self, nums: List[int]) -> int:
        cnt = Counter(nums)
        freq_of_freq = Counter(cnt.values())
        for x in nums:
            if freq_of_freq[cnt[x]] == 1:
                return x
        return -1

Complexity

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Time O(n), space O(n).

Mistakes to Avoid

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• “First” follows the original array’s order, not cnt’s.

Interview Explanation

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I count each value, then count how many values share each count. Scanning the original array from the left, the first element whose count belongs to only one value is the answer. Time O(n).