How do you solve First Element with Unique Frequency on LeetCode?

First Element with Unique Frequency (LeetCode #3843) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: Unique frequency means no other number shares that count.

What is the time complexity of First Element with Unique Frequency?

The optimal solution for First Element with Unique Frequency runs in O(n) time and uses O(n) space. Two passes through the array yield linear time complexity, with additional space for hash maps.

What is the brute force solution for First Element with Unique Frequency?

The brute force approach for First Element with Unique Frequency is Brute Force, with O(n²) time complexity and O(1) space complexity. Check the frequency of each element against all others. This is straightforward but inefficient. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve First Element with Unique Frequency?

First Element with Unique Frequency uses the following concepts: Array, Hash Table, Counting. The recommended patterns to study are: Hash Map, Array.

What are the interview tips for First Element with Unique Frequency?

Clarify the problem requirements. Discuss potential edge cases. Explain your thought process clearly.

What are common mistakes when solving First Element with Unique Frequency?

Counting frequencies incorrectly. Not considering the order of elements.

#3843

First Element with Unique Frequency

Medium

ArrayHash TableCountingHash MapArray

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

💡

Intuition

Time O(n)Space O(n)

Use two hash maps to count frequencies and track unique counts efficiently.

⚙️

Algorithm

3 steps

1Step 1: Count the frequency of each number using a hash map.
2Step 2: Count how many numbers have each frequency using a second hash map.
3Step 3: Iterate through the original array to find the first number with a unique frequency.

solution.py8 lines

1def firstUnique(nums):
2    from collections import Counter
3    freq = Counter(nums)
4    freqCount = Counter(freq.values())
5    for num in nums:
6        if freqCount[freq[num]] == 1:
7            return num
8    return -1

ℹ

Complexity note: Two passes through the array yield linear time complexity, with additional space for hash maps.

1Unique frequency means no other number shares that count.
2Order of appearance matters for the final output.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.