Is Count Digit Appearances easy or hard?

Most platforms classify this problem as Medium. The brute force idea is simple, but deriving the O(log n) positional formula requires understanding how digits repeat across number ranges.

Count Digit Appearances Python/Java solution

Both Python and Java implementations follow the same positional counting logic. Use a loop with factor *= 10 and compute higher, current, and lower values at each step. The algorithm runs in O(log n) time and uses constant extra space.

How to solve Count Digit Appearances in O(log n)?

Break the number into positional components using a factor (1, 10, 100...). For each position, compute higher = n / (factor * 10), current = (n / factor) % 10, and lower = n % factor. These values determine how many full cycles contribute occurrences of the target digit at that position.

What is the best approach for Count Digit Appearances?

The positional digit counting method is the most efficient approach. It analyzes each decimal position (ones, tens, hundreds) and calculates how many times the target digit appears using higher, current, and lower digit segments. This reduces the complexity to O(log n) time with constant space.

Is Count Digit Appearances asked at Google/Amazon/Meta?

Digit counting and positional analysis problems appear frequently in interviews at companies like Google and Amazon. Variants such as counting the number of digit '1's from 1 to n or counting digits in a range test mathematical reasoning and edge‑case handling.

What data structure is used in Count Digit Appearances?

The optimal approach relies on mathematical decomposition rather than complex data structures. Only a few integer variables are required to track higher digits, the current digit, and lower digits while iterating through each decimal position.

What is the time complexity of Count Digit Appearances?

The optimal solution runs in O(log n) time because it processes each digit position of the number once. Since an integer n has log10(n) digits, the algorithm iterates through those positions and performs constant-time calculations per position.

3895. Count Digit Appearances - Practice Online

You are given an integer array nums and an integer digit.

Return the total number of times digit appears in the decimal representation of all elements in nums.

Example 1:

Input: nums = [12,54,32,22], digit = 2

Output: 4

Explanation:

The digit 2 appears once in 12 and 32, and twice in 22. Thus, the total number of times digit 2 appears is 4.

Example 2:

Input: nums = [1,34,7], digit = 9

Output: 0

Explanation:

The digit 9 does not appear in the decimal representation of any element in nums, so the total number of times digit 9 appears is 0.

Constraints:

1 <= nums.length <= 1000
1 <= nums[i] <= 10⁶
0 <= digit <= 9