Is Largest Unique Number easy or hard?

Largest Unique Number is categorized as an Easy problem. It mainly tests familiarity with arrays and hash table frequency counting, a foundational technique used across many coding interview questions.

Largest Unique Number Python/Java solution

Most implementations follow the same pattern: count frequencies using a dictionary (Python) or HashMap (Java), then find the largest key with count equal to one. The algorithm remains O(n) regardless of language.

How to solve Largest Unique Number in O(n)?

Iterate through the array and store counts in a hash map or frequency array. After building the counts, check each value and track the maximum number whose count equals one. Hash lookups are O(1), so the entire algorithm completes in linear time.

What is the best approach for Largest Unique Number?

The most efficient approach uses frequency counting with a hash map. First count how many times each value appears, then scan for the largest number whose frequency equals one. This runs in O(n) time with O(n) space and avoids the O(n log n) cost of sorting.

Is Largest Unique Number asked at Google/Amazon/Meta?

Problems involving frequency counting and hash maps appear frequently in interviews at companies like Amazon, Meta, and Google. While this exact problem may vary, the underlying pattern—tracking element frequencies to detect uniqueness—is very common.

What data structure is used in Largest Unique Number?

The typical solution uses a hash table (dictionary or map) to store frequencies of numbers. If the value range is small, a fixed-size counting array can replace the hash map for faster lookups and lower overhead.

What is the time complexity of Largest Unique Number?

The optimal solution runs in O(n) time because each element is processed during the counting pass and checked once more when determining the largest unique value. Space complexity is O(n) in the worst case due to the frequency map storing distinct elements.

Description Hints 3 Solution Video 10:17

1133. Largest Unique Number

Easy71.3% acceptance1 companyPremium elsewhere, free here

Given an integer array nums, return the largest integer that only occurs once. If no integer occurs once, return -1.

Example 1:

Input: nums = [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 is the answer.

Example 2:

Input: nums = [9,9,8,8]
Output: -1
Explanation: There is no number that occurs only once.

Constraints:

1 <= nums.length <= 2000
0 <= nums[i] <= 1000

Asked at 1 company

Amazon

Topics

Hints open one at a time

Hint 1

Find the number of occurrences of each value.

Hint 2

Use an array or a hash table to do that.

Hint 3

Look for the largest value with number of occurrences = 1.

10:17

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Starter codePython 3Spaces: 4

Examples

Case 1Case 2

Input

[5,7,3,9,4,9,8,3,1]

Input

[9,9,8,8]

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