Applying Sorting and Two-Pointer Technique in C#

Hello there! Are you ready to solve another engaging problem today? We have a practical task that will enhance your problem-solving skills. It involves critical aspects of programming — dealing with arrays and using techniques such as sorting and the two-pointer method. So, let's jump in!

Our task is as follows. Suppose you have two equally long arrays, A and B, with lengths varying from 1 to 1000, with each element being a unique positive integer ranging from $1$ up to $10^6$. Your challenge is to create a C# method that performs the following steps:

1. For each element B[i] in array B, double its value to get 2 * B[i].
2. Find the closest number to 2 * B[i] in array B. Let's call this closest number B[j].
3. For each index i in array B, get the value at index j in array A, i.e., A[j].
4. Create a new array where each element is A[j] corresponding to the closest number found in B.

To illustrate this, let's consider an example. We have:

int[] A = { 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 };
int[] B = { 4, 12, 3, 9, 6, 1, 5, 8, 37, 25, 100 };

After running your method, the resulting array should look like this:

int[] result = { 80, 100, 50, 20, 20, 60, 40, 20, 110, 90, 110 };

Let's walk through the first few steps:

The first item in B is 4 at index=0. Doubling this number gives us 8. The closest number to 8 in array B is 8, which is at index=7. The number at the same index in array A is 80, so we add 80 to our new array.

The second item in B is 12 at index=1. Doubling this number gives us 24. The closest number to 24 in B is 25, which is at index=9. The corresponding index in A has the number 100. So, we add 100 to our new array.

The third item in B is 3 at index=2. Doubling this number gives us 6. The closest number to 6 in B is 6, which is at index=4. The corresponding index in A has the number 50. So, we add 50 to our new array.

We continue this process for the rest of the elements in B.

Let's embark on our solution-building journey by constructing a sorted list for array B. This list will include pairs of values (val) and their corresponding indices (idx) from array B. Here, val represents the element in B, while idx denotes the index at which val is found in array B.

This sorted list will be similar to an associative array, storing 'value-index' pairs. It not only organizes the data for efficient retrieval but also makes it easier for us to traverse the list. Here's the introductory part of our C# method, including the creation of the sorted list:

using System;
using System.Collections.Generic;

public class Solution
{
    public static int[] FindAndReplace(int[] A, int[] B)
    {
        List<(int value, int index)> B_sorted = new List<(int, int)>();
        for (int i = 0; i < B.Length; i++)
        {
            B_sorted.Add((B[i], i));
        }
        B_sorted.Sort((a, b) => a.value.CompareTo(b.value));

In the above code, we generate a list of tuples comprising the values from B and their respective indices using a simple loop. Then, the Sort method arranges these tuples in ascending order of their values.