Count Digit-Reversal Equivalent Pairs

Question

The question asks the candidate to determine how many pairs of numbers in a given list are digit-reversal equivalents, meaning that one number is the reverse of the other.

Codemia · Accepted Answer

To solve the problem of counting digit-reversal equivalent pairs, we need to understand that two numbers are considered equivalent if one is the reverse of the other. For example, the pair (123, 321) would be considered equivalent. Our goal is to count how many such pairs exist in the provided list of numbers.

We can take the following approach to solve this problem:
1. Create a set to store the reversed versions of each number.
2. Iterate through the list of numbers, reverse each number, and check if the reversed number is already in the set.
3. If it is, we have found a pair; if not, we add the reversed number to the set.
4. Keep a count of the pairs found during the iteration.

Here is a Python implementation of the described approach:
```python
def count_digit_reversal_pairs(nums):
    seen = set()
    count = 0
    for num in nums:
        reversed_num = int(str(num)[::-1])
        if reversed_num in seen:
            count += 1
        seen.add(num)
    return count
```
This function iterates through the list of numbers, reverses each number, and checks for pairs efficiently using a set.

The time complexity of this solution is O(n), where n is the number of elements in the input list. This is because we are making a single pass through the list. The space complexity is also O(n) due to the storage of seen numbers in a set.

We should consider edge cases such as:
- An empty list, which should return 0.
- Lists with single elements, which should also return 0 since no pairs can be formed.
- Negative numbers, which may require additional handling if we want to consider their reversals.

Count Digit-Reversal Equivalent Pairs

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