Is Number of Unequal Triplets in Array easy or hard?

LeetCode classifies this problem as Easy. The brute force solution is straightforward, but the intended optimization introduces a common interview pattern: counting combinations using frequency maps to avoid unnecessary nested loops.

Number of Unequal Triplets in Array Python/Java solution

In Python or Java, the common solution builds a frequency map using a dictionary or HashMap. Then iterate through the map while maintaining counts of elements to the left and right to compute left * freq * right. This approach runs in O(n) time and is concise in both languages.

How to solve Number of Unequal Triplets in Array in O(n)?

Count the frequency of each number using a hash map. Iterate through the distinct values while tracking how many elements are on the left and right of the current value group. The number of valid triplets formed with that group is left * freq * right. Summing these counts produces the answer in linear time.

What is the best approach for Number of Unequal Triplets in Array?

The best approach uses frequency counting with a hash map. First count how many times each number appears, then compute combinations using leftCount * freq * rightCount for each unique value group. This avoids checking every index triplet and runs in O(n + k) time where k is the number of distinct values.

Is Number of Unequal Triplets in Array asked at Google/Amazon/Meta?

Triplet counting and frequency-based array problems appear frequently in interviews at companies like Amazon, Google, and Meta. Variations often test whether you can reduce nested loops by grouping elements with a hash map or sorting strategy.

What data structure is used in Number of Unequal Triplets in Array?

The optimized solution uses a hash map (or dictionary) to store frequencies of each number. This allows constant-time lookups and enables counting combinations of distinct value groups instead of iterating over index triplets.

What is the time complexity of Number of Unequal Triplets in Array?

The brute force method takes O(n^3) time because it checks every possible triplet of indices. The optimized frequency counting approach runs in O(n + k) time and O(k) space, where k is the number of unique elements. A sorting-based alternative runs in O(n log n).

Description Hints 2 Solution Video 7:33

2475. Number of Unequal Triplets in Array

Easy73.3% acceptance1 company

You are given a 0-indexed array of positive integers nums. Find the number of triplets (i, j, k) that meet the following conditions:

0 <= i < j < k < nums.length
nums[i], nums[j], and nums[k] are pairwise distinct.
- In other words, nums[i] != nums[j], nums[i] != nums[k], and nums[j] != nums[k].

Return the number of triplets that meet the conditions.

Example 1:

Input: nums = [4,4,2,4,3]
Output: 3
Explanation: The following triplets meet the conditions:
- (0, 2, 4) because 4 != 2 != 3
- (1, 2, 4) because 4 != 2 != 3
- (2, 3, 4) because 2 != 4 != 3
Since there are 3 triplets, we return 3.
Note that (2, 0, 4) is not a valid triplet because 2 > 0.

Example 2:

Input: nums = [1,1,1,1,1]
Output: 0
Explanation: No triplets meet the conditions so we return 0.

Constraints:

3 <= nums.length <= 100
1 <= nums[i] <= 1000

Asked at 1 company

Paytm

Topics

Hints open one at a time

Hint 1

The constraints are very small. Can we try every triplet?

Hint 2

Yes, we can. Use three loops to iterate through all the possible triplets, ensuring the condition i < j < k holds.

7:33

Still stuck?Read 6 approaches, or watch a walkthrough.

Open solution