# 19. Rearranging an Array with O(1) Extra Space

The problem can be found at the following link: [Question Link](https://practice.geeksforgeeks.org/problems/rearrange-an-array-with-o1-extra-space3142/1)

## My Approach

To rearrange an array with O(1) extra space, I have used the following approach:

* Given the constraint that the maximum value of `n` is `10^5`, along with a `long long` array called `arr`, we can leverage this constraint to store both the old and new numbers within the same array elements.
* Each array element will consist of a 10-digit number, where the first 5 digits represent the new number obtained after rearrangement, and the last 5 digits represent the old number.
* Initially, we set both the new and old numbers at the same index during the iteration. To identify the new number, we consider the first 5 digits of the 10-digit number and obtain the desired number by dividing it by `10^5`.

## Example:

### Input:

```
10
2 8 4 1 9 5 3 0 7 6
```

We can store the array numbers as:

```
400002 700008 900004 800001 600009 500005 100003 200000 7 300006
```

By dividing these numbers by `10^5`, we obtain the desired values:

### Output:

```
4 7 9 8 6 5 1 2 0 3
```

## Time and Auxiliary Space Complexity

* **Time Complexity**: `O(N)`
* **Auxiliary Space Complexity**: `O(1)`

## Code (C++)

```cpp
class Solution{
    public:
    void arrange(long long arr[], int n) {
        long long offset = 1e5;
        for (int i = 0; i < n; i++) {
            arr[i] = (arr[arr[i]] % offset) * offset + arr[i];
        }
        // Set the array with new value as "newValue = newvalue*offset + oldValue"

        for (int i = 0; i < n; i++) {
            arr[i] = arr[i] / offset;
        }
    }
};
```

## Contribution and Support

For discussions, questions, or doubts related to this solution, please visit our [discussion section](https://github.com/getlost01/gfg-potd/discussions). We welcome your input and aim to foster a collaborative learning environment.

If you find this solution helpful, consider supporting us by giving a `⭐ star` to the [getlost01/gfg-potd](https://github.com/getlost01/gfg-potd) repository.


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://gl01.gitbook.io/gfg-editorials/2023/06-2023-june-18/19-rearrange-an-array-with-o-1-extra-space.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.