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In-Place algorithms
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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; }

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Output
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3 2 1

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Out-of-Place algorithms
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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; }

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Output
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3 2 1

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