Find The Least Frequent Digit

Find The Least Frequent Digit

What is this problem about?

The Find The Least Frequent Digit coding problem asks you to analyze a list of integers and identify which digit (0-9) appears the fewest number of times across all numbers in the collection. If there is a tie, usually the smallest digit is returned. You only consider digits that appear at least once.

Why is this asked in interviews?

Flipkart uses this "Easy" question to test basic programming logic and Hash Table interview patterns. It evaluates your ability to extract digits from numbers and manage a frequency map. It’s a baseline test of your ability to handle counts and apply simple tie-breaking rules, which are common tasks in data processing and analytics.

Algorithmic pattern used

This is a Frequency Counting problem.

1. Initialize an array of size 10 (or a Hash Map) to store counts for digits 0-9.
2. Iterate through each number in the input array.
3. For each number, use a while loop with % 10 and / 10 to extract every digit.
4. Increment the corresponding index in your frequency array.
5. After processing all numbers, iterate through the frequency array to find the smallest non-zero count.

Example explanation

Input: [12, 23, 31]

1. Digits: 1, 2, 2, 3, 3, 1.
2. Counts: {1: 2, 2: 2, 3: 2}.
3. All digits (1, 2, 3) appear twice.
4. Tie-breaker: Return the smallest digit, which is 1.

Common mistakes candidates make

• String Conversion: Converting every number to a string to count digits. This works but is less efficient than mathematical digit extraction.
• Handling Zero counts: Incorrectly identifying a digit that never appeared as the "least frequent" instead of only looking at digits that appeared at least once.
• Tie-breaking: Returning the frequency instead of the digit itself.

Interview preparation tip

For all digit-related problems, have the while(n > 0) { digit = n % 10; n /= 10; } snippet ready. It is faster and more memory-efficient than string-based approaches and is a standard Math interview pattern.