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Nilesh Raut

Posted on • Updated on • Originally published at nileshblog.tech

# Cracking the Code: Counting All Valid Pickup and Delivery Options

## Unveiling the Mysteries of Leetcode 1359

We've all been there – staring at a coding problem that seems to defy logic, with brackets, numbers, and letters all dancing in a seemingly incomprehensible pattern. But fear not, because today, we're diving headfirst into the intriguing world of Leetcode 1359Count All Valid Pickup and Delivery Options (HARD).

## Deconstructing the Challenge

Let's begin by dissecting the problem itself. In Leetcode 1359, you're given n orders to deliver, represented as a string of 'P' and 'D'. 'P' stands for pickup, and 'D' stands for delivery. The catch? You must fulfill these orders in such a way that they are all valid. Now, what exactly does 'valid' mean here? A valid delivery order must follow these rules:

1. Each delivery order must be preceded by its corresponding pickup order.
2. No order can be dropped off before it's been picked up.
3. All orders must be delivered. Sounds challenging, right? Well, fear not – we're here to unravel this puzzle step by step.

## The Power of Permutations

Let's think of it like this: each pair of 'P' and 'D' is like a dynamic duo, a superhero and their sidekick. The challenge is to find all the possible ways these duos can be arranged while adhering to the rules.
If you've ever played with building blocks as a child, think of the 'P's as the blocks you pick up, and the 'D's as the places you can stack them. Now, imagine how many different ways you can stack these blocks while ensuring that each 'P' has a corresponding 'D' and none are left unstacked. That's the essence of what we're trying to solve here – counting all the valid permutations.

## The Beauty of Combinatorics

To tackle this problem, we delve into the realm of combinatorics. Combinatorics is like the art of arranging things in a specific order, and in this case, it's about arranging 'P's and 'D's in valid pairs.
We can break down the process into smaller steps:

1. Calculate the number of ways to arrange 'P's among themselves – this is usually represented as P(n).
2. Calculate the number of ways to arrange 'D's among themselves – represented as D(n).
3. Multiply P(n) and D(n) to get the total valid permutations.

## The Devil in the Details

Of course, like all good coding problems, the devil is in the details. When dealing with permutations, we must consider factorials – the product of all positive integers up to a given number. So, P(n) is essentially n factorial (n!), and the same goes for D(n).
In other words, P(n) = n! and D(n) = n!. Therefore, to find the total valid permutations, we simply multiply n! by n! – that's (n!)^2.

## A Quick Recap

So, to sum it up, in Leetcode 1359 – Count All Valid Pickup and Delivery Options (HARD), our goal is to count all the valid permutations of 'P's and 'D's. We achieve this by calculating the factorial of n, which represents the number of orders. Then, we square this value to get the total valid permutations.
This problem is a great exercise in combinatorics and understanding how to approach complex coding challenges. Remember, it's not about solving it instantly but about breaking it down into manageable steps.
So, next time you encounter a seemingly insurmountable coding problem, don't be discouraged – think of it as a puzzle waiting to be solved, one piece at a time. Happy coding!

## Thank You for Joining Us

We hope you found this exploration of Leet code 1359 insightful and helpful. If you have any questions or want to delve deeper into coding challenges like this one, feel free to reach out. Until next time, happy coding!