Is Unique 3-Digit Even Numbers easy or hard?

Unique 3-Digit Even Numbers is classified as an Easy problem. The main challenge is correctly applying the constraints: no leading zero, digits used once per number, and the final digit must be even. Once those checks are implemented, the enumeration solution is straightforward.

Unique 3-Digit Even Numbers Python/Java solution

In Python or Java, iterate over all index triples, skip cases where the first digit is 0 or the last digit is odd, and build the number using arithmetic (hundreds, tens, ones). Insert each valid number into a HashSet or set. Finally convert the set to a sorted list or array before returning.

How to solve Unique 3-Digit Even Numbers in O(n)?

An O(n) solution is generally not feasible because the algorithm must consider combinations of three digits. The practical approach enumerates triples of indices with O(n^3) time. Early filtering (skip leading zero and enforce even last digit) reduces unnecessary checks but does not change the asymptotic complexity.

What is the best approach for Unique 3-Digit Even Numbers?

The most practical approach is Hash Set + Enumeration. Iterate over all triples of indices to construct three-digit numbers while enforcing constraints such as no leading zero and an even last digit. Insert each valid number into a hash set to guarantee uniqueness. This approach runs in O(n^3) time and O(k) space.

Is Unique 3-Digit Even Numbers asked at Google/Amazon/Meta?

Problems like this frequently appear in coding interviews at large tech companies because they test enumeration, constraint filtering, and hash set usage. Variants involving digit permutations and uniqueness checks are common in interview question banks used by companies such as Amazon and Google.

What data structure is used in Unique 3-Digit Even Numbers?

A hash set is the main data structure used to store generated numbers and automatically remove duplicates. The digits are provided in an array, and nested iteration enumerates valid combinations while the set guarantees uniqueness.

What is the time complexity of Unique 3-Digit Even Numbers?

The typical solution runs in O(n^3) time because three nested loops enumerate all possible digit positions. Since the number length is fixed at three, the cubic complexity is acceptable. Space complexity is O(k) for storing unique results in a hash set.

Description Hints 1 Solution Video 14:51

3483. Unique 3-Digit Even Numbers

Easy79.8% acceptance3 companies

You are given an array of digits called digits. Your task is to determine the number of distinct three-digit even numbers that can be formed using these digits.

Note: Each copy of a digit can only be used once per number, and there may not be leading zeros.

Example 1:

Input: digits = [1,2,3,4]

Output: 12

Explanation: The 12 distinct 3-digit even numbers that can be formed are 124, 132, 134, 142, 214, 234, 312, 314, 324, 342, 412, and 432. Note that 222 cannot be formed because there is only 1 copy of the digit 2.

Example 2:

Input: digits = [0,2,2]

Output: 2

Explanation: The only 3-digit even numbers that can be formed are 202 and 220. Note that the digit 2 can be used twice because it appears twice in the array.

Example 3:

Input: digits = [6,6,6]

Output: 1

Explanation: Only 666 can be formed.

Example 4:

Input: digits = [1,3,5]

Output: 0

Explanation: No even 3-digit numbers can be formed.

Constraints:

3 <= digits.length <= 10
0 <= digits[i] <= 9

Asked at 3 companies

Google Amazon Bloomberg

Topics

Hints open one at a time

Hint 1

Use brute force to try all possibilities

14:51

Still stuck?Read the written solution, or watch a walkthrough.

Open solution