Program to sorting important mails from different mailboxes in Python

Suppose we have a list of mailboxes where each mailbox contains a list of strings. Each string represents either "J" for junk, "P" for personal, or "W" for work emails. We need to process each mailbox in round-robin order starting from the first mailbox, filter out junk emails, and return a single sorted list.

So, if the input is like mailboxes = [["W", "P"],["J", "P", "J"],["W"]], then the output will be ["W", "W", "P", "P"]. In processing order without filtering, we have W ? J ? W ? P ? P ? J. After filtering out junk emails, we get W ? W ? P ? P.

Algorithm

To solve this problem, we will follow these steps ?

• Initialize the number of mailboxes
• Create a result list to store important mails
• Create a counts array to track current position in each mailbox
• Use round-robin processing to iterate through all mailboxes
• Filter out junk mails and add important ones to result

Example

Let us see the following implementation to get better understanding ?

class Solution:
    def solve(self, mailboxes):
        n_mailboxes = len(mailboxes)
        result = []
        counts = [0] * n_mailboxes
        more = True
        
        while more:
            more = False
            for i in range(n_mailboxes):
                index, mailbox = counts[i], mailboxes[i]
                if index < len(mailbox):
                    more = True
                    counts[i] += 1
                    mail = mailbox[index]
                    if mail != "J":
                        result.append(mail)
        return result

# Test the solution
ob = Solution()
mailboxes = [["W", "P"],["J", "P", "J"],["W"]]
print(ob.solve(mailboxes))

['W', 'W', 'P', 'P']

How It Works

The algorithm processes mailboxes in round-robin fashion:

• Round 1: Takes first email from each mailbox: "W" (mailbox 0), "J" (mailbox 1 - filtered), "W" (mailbox 2)
• Round 2: Takes second email: "P" (mailbox 0), "P" (mailbox 1), no more emails in mailbox 2
• Round 3: Only mailbox 1 has remaining email: "J" (filtered out)

Alternative Approach Using Queue

We can also solve this using a more Pythonic approach with iterators ?

from itertools import chain, zip_longest

def sort_important_mails(mailboxes):
    # Transpose mailboxes to get round-robin order
    rounds = zip_longest(*mailboxes, fillvalue=None)
    
    # Flatten and filter out None and "J"
    result = []
    for round_mails in rounds:
        for mail in round_mails:
            if mail and mail != "J":
                result.append(mail)
    
    return result

# Test the solution
mailboxes = [["W", "P"],["J", "P", "J"],["W"]]
print(sort_important_mails(mailboxes))

['W', 'W', 'P', 'P']

Comparison

Where n is total number of emails and m is number of mailboxes.

Conclusion

The round-robin approach efficiently processes multiple mailboxes while filtering junk emails. The zip_longest method provides a more Pythonic solution with better readability for the same functionality.