Is Count Pairs That Form a Complete Day I easy or hard?

LeetCode classifies this problem as Easy. The brute force approach is straightforward, and the optimized solution requires recognizing the modulo pairing pattern. Understanding remainder complements is the main insight.

Count Pairs That Form a Complete Day I Python/Java solution

Most implementations compute hours[i] % 24 and maintain a frequency array or hash map. Python commonly uses a list of size 24, while Java and C++ often use arrays or HashMap structures. The algorithm stays the same across languages with O(n) time complexity.

How to solve Count Pairs That Form a Complete Day I in O(n)?

Compute the remainder of each hour value with 24. Maintain a frequency array or hash map for these remainders. For each number, calculate the complement remainder needed to reach 24 and add its frequency to the pair count. Update the frequency of the current remainder afterward.

What is the best approach for Count Pairs That Form a Complete Day I?

The optimal approach uses remainder counting with modulo 24. Convert each value to hours % 24 and track frequencies of remainders. For each element, look for the complement remainder (24 - r) % 24 that would complete a full day. This reduces the complexity to O(n) time with constant space.

Is Count Pairs That Form a Complete Day I asked at Google/Amazon/Meta?

Problems involving remainder pairing and hash-map counting frequently appear in interviews at companies like Amazon, Google, and Meta. While this exact question may vary, the underlying pattern—pair sums with modular arithmetic—is a common interview topic.

What data structure is used in Count Pairs That Form a Complete Day I?

The optimized solution uses a hash table or a fixed-size frequency array of length 24. This structure tracks how many times each remainder has appeared so far, enabling constant-time complement lookups.

What is the time complexity of Count Pairs That Form a Complete Day I?

The brute force approach runs in O(n²) time because every pair of elements is checked. The optimized solution processes each element once and performs constant-time hash lookups, resulting in O(n) time complexity and O(1) space due to only 24 possible remainders.

Description Hints 1 Solution Video 11:42

3184. Count Pairs That Form a Complete Day I

Easy78.0% acceptance1 company

Given an integer array hours representing times in hours, return an integer denoting the number of pairs i, j where i < j and hours[i] + hours[j] forms a complete day.

A complete day is defined as a time duration that is an exact multiple of 24 hours.

For example, 1 day is 24 hours, 2 days is 48 hours, 3 days is 72 hours, and so on.

Example 1:

Input: hours = [12,12,30,24,24]

Output: 2

Explanation:

The pairs of indices that form a complete day are (0, 1) and (3, 4).

Example 2:

Input: hours = [72,48,24,3]

Output: 3

Explanation:

The pairs of indices that form a complete day are (0, 1), (0, 2), and (1, 2).

Constraints:

1 <= hours.length <= 100
1 <= hours[i] <= 10⁹

Asked at 1 company

Infosys

Topics

Hints open one at a time

Hint 1

Brute force all pairs (i, j) and check if they form a valid complete day. It is considered a complete day if (hours[i] + hours[j]) % 24 == 0.

11:42

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