Introduction

In this blog post, we will explore how to solve the LeetCode problem "3185" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

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 <= 5 * 105 1 <= hours[i] <= 109

Explanation

Here's a breakdown of the solution:

• Core Idea: The problem boils down to finding pairs (i, j) such that hours[i] + hours[j] is divisible by 24. We can use the modulo operator (%) to efficiently check for divisibility.
• Optimization with Hash Map (Counter): We can improve efficiency by using a hash map (specifically, a Counter in Python) to store the counts of each hours[i] % 24. This allows us to quickly find the number of elements that, when added to hours[i] % 24, result in a multiple of 24.

• Avoiding Double Counting: Since we need to consider pairs (i, j) where i < j, we process each element one at a time, updating the count after calculating the number of valid pairs it forms with the elements processed so far.

• Time & Space Complexity: O(n), where n is the length of the hours array, because we iterate through the array once. Space complexity is O(1) because the counter stores at most 24 different keys (the possible remainders when dividing by 24).

Code

    from collections import Counter

def complete_days(hours):
    """
    Calculates the number of pairs (i, j) where i < j and hours[i] + hours[j]
    is a multiple of 24.

    Args:
        hours: A list of integers representing times in hours.

    Returns:
        The number of pairs forming complete days.
    """

    count = 0
    remainders = Counter()

    for hour in hours:
        remainder = hour % 24
        complement = (24 - remainder) % 24  # Find the remainder that, when added to current remainder, results in 24.
        count += remainders[complement] # Add number of existing remainders which complement current remainder

        remainders[remainder] += 1 # Update the count

    return count