In-place vs out-of-place algorithms

In-Place algorithms

This is a category of the algorithms that do not consume any extra space in order to solve a given task. They generally override the given input with the output. We can say that the auxiliary space complexity of these algorithms is O(1). These algorithms may sometimes require a very small space but that space should not depend on size of input. The algorithms that work recursively and require call stack memory are generally not considered in-place algorithms.

Below is an example of an in-place algorithm to reverse a given array:

#include <bits/stdc++.h>
using namespace std;

void reverseArray(int arr[], int start, int end)
{
    while (start < end)
    {
        int temp = arr[start];
        arr[start] = arr[end];
        arr[end] = temp;
        start++;
        end--;
    }
}   

int main()
{
    int arr[] = {1, 2, 3};

    int n = sizeof(arr) / sizeof(arr[0]);   

    reverseArray(arr, 0, n-1);

    for (int i = 0; i < n; i++)
        cout << arr[i] << " ";

    return 0;
}

Output

3 2 1

Out-of-Place algorithms

These algorithms require extra memory to accomplish a given task. The time complexity of these algorithms is never constant and depends on the size of the input. Below is an algorithm to reverse a given array using extra space. The auxiliary space complexity of the algorithm is O(N).

#include <iostream>
using namespace std;
int main()
{
    int original_arr[] = {1, 2, 3};

    int len = sizeof(original_arr)/sizeof(original_arr[0]);

    int copied_arr[len], i, j;

    for (i = 0; i < len; i++) 
    {
        copied_arr[i] = original_arr[len - i - 1];
   }
    for(int i=0;i<len;i++) 
    {
        cout<<copied_arr[i]<<" ";
    }
    return 0;
}

Output

3 2 1

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