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# Minimum array elements to be changed to make Recaman’s sequence

• Last Updated : 12 May, 2021

Given an array arr[] of N elements. The task is to find the minimum number of elements to be changed in the array such that the array contains first N Recaman’s Sequence terms. Note that Recaman terms may be present in any order in the array.
First few terms of Recaman’s Sequence are:

0, 1, 3, 6, 2, 7, 13, 20, 12, 21, 11, 22, 10, 23, 9, 24, 8, …..

Examples:

Input: arr[] = {44, 0, 2, 3, 9}
Output:
N = 5 and first 5 Recaman Numbers are 0, 1, 3, 6 and 2
44 and 9 must be replaced with 6 and 1
Hence 2 changes are required.
Input: arr[] = {0, 33, 3, 1}
Output:

Approach:

• Insert first N Recaman’s Sequence terms in a set.
• Traverse the array from left to right and check if array element is present in the set.
• If current element is present in the set that remove it from the set.
• Minimum changes required is the size of the final reduced set.

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach``#include ``using` `namespace` `std;` `int` `recamanGenerator(``int` `arr[], ``int` `n)``{``    ``// First term of the sequence is always 0``    ``arr = 0;` `    ``// Fill remaining terms using recursive``    ``// formula``    ``for` `(``int` `i = 1; i <= n; i++) {``        ``int` `temp = arr[i - 1] - i;``        ``int` `j;` `        ``for` `(j = 0; j < i; j++) {` `            ``// If arr[i-1] - i is negative or``            ``// already exists``            ``if` `((arr[j] == temp) || temp < 0) {``                ``temp = arr[i - 1] + i;``                ``break``;``            ``}``        ``}` `        ``arr[i] = temp;``    ``}``}` `// Function that returns minimum changes required``int` `recamanArray(``int` `arr[], ``int` `n)``{` `    ``// Set to store first n Recaman numbers``    ``unordered_set<``int``> s;` `    ``// Generate and store``    ``// first n Recaman numbers``    ``int` `recaman[n];``    ``recamanGenerator(recaman, n);` `    ``// Insert first n Recaman numbers to set``    ``for` `(``int` `i = 0; i < n; i++)``        ``s.insert(recaman[i]);` `    ``for` `(``int` `i = 0; i < n; i++) {` `        ``// If current element of the array``        ``// is present in the set``        ``auto` `it = s.find(arr[i]);``        ``if` `(it != s.end())``            ``s.erase(it);``    ``}` `    ``// Return the remaining number of``    ``// elements in the set``    ``return` `s.size();``}` `// Driver code``int` `main()``{` `    ``int` `arr[] = { 7, 11, 20, 4, 2, 1, 8, 6 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);` `    ``cout << recamanArray(arr, n);` `    ``return` `0;``}`

## Java

 `// Java implementation of the approach``import` `java.util.*;` `class` `GFG``{` `static` `int` `recamanGenerator(``int` `arr[], ``int` `n)``{``    ``// First term of the sequence is always 0``    ``arr[``0``] = ``0``;` `    ``// Fill remaining terms using recursive``    ``// formula``    ``for` `(``int` `i = ``1``; i <= n; i++)``    ``{``        ``int` `temp = arr[i - ``1``] - i;``        ``int` `j;` `        ``for` `(j = ``0``; j < i; j++)``        ``{` `            ``// If arr[i-1] - i is negative or``            ``// already exists``            ``if` `((arr[j] == temp) || temp < ``0``)``            ``{``                ``temp = arr[i - ``1``] + i;``                ``break``;``            ``}``        ``}` `        ``arr[i] = temp;``    ``}``    ``return` `0``;``}` `// Function that returns minimum changes required``static` `int` `recamanArray(``int` `arr[], ``int` `n)``{``    `  `    ``// Set to store first n Recaman numbers``    ``Set s=``new` `HashSet();` `    ``// Generate and store``    ``// first n Recaman numbers``    ``int` `recaman[]=``new` `int``[n+``1``];``    ``recamanGenerator(recaman, n);` `    ``// Insert first n Recaman numbers to set``    ``for` `(``int` `i = ``0``; i < n; i++)``        ``s.add(recaman[i]);` `    ``for` `(``int` `i = ``0``; i < n; i++)``    ``{``        ``// If current element of the array``        ``// is present in the set``        ``if` `(s.contains(arr[i]))``            ``s.remove(arr[i]);``    ``}` `    ``// Return the remaining number of``    ``// elements in the set``    ``return` `s.size();``}` `// Driver code``public` `static` `void` `main(String args[])``{` `    ``int` `arr[] = { ``7``, ``11``, ``20``, ``4``, ``2``, ``1``, ``8``, ``6` `};``    ``int` `n = arr.length;` `    ``System.out.print( recamanArray(arr, n));``}``}` `// This code is contributed by Arnab Kundu`

## Python3

 `# Python3 implementation of the approach``def` `recamanGenerator(arr, n):``    ` `    ``# First term of the sequence``    ``# is always 0``    ``arr[``0``] ``=` `0` `    ``# Fill remaining terms using``    ``# recursive formula``    ``for` `i ``in` `range``(``1``, n):``        ``temp ``=` `arr[i ``-` `1``] ``-` `i``        ``j ``=` `0` `        ``for` `j ``in` `range``(i):` `            ``# If arr[i-1] - i is negative or``            ``# already exists``            ``if` `((arr[j] ``=``=` `temp) ``or` `temp < ``0``):``                ``temp ``=` `arr[i ``-` `1``] ``+` `i``                ``break` `        ``arr[i] ``=` `temp` `# Function that returns minimum``# changes required``def` `recamanArray(arr, n):` `    ``# Set to store first n Recaman numbers``    ``s ``=` `dict``()` `    ``# Generate and store``    ``# first n Recaman numbers``    ``recaman ``=` `[``0` `for` `i ``in` `range``(n)]``    ``recamanGenerator(recaman, n)` `    ``# Insert first n Recaman numbers to set``    ``for` `i ``in` `range``(n):``        ``s[recaman[i]] ``=` `s.get(recaman[i], ``0``) ``+` `1` `    ``for` `i ``in` `range``(n):` `        ``# If current element of the array``        ``# is present in the set``        ``if` `arr[i] ``in` `s.keys():``            ``del` `s[arr[i]]` `    ``# Return the remaining number of``    ``# elements in the set``    ``return` `len``(s)` `# Driver code``arr ``=` `[``7``, ``11``, ``20``, ``4``, ``2``, ``1``, ``8``, ``6` `]``n ``=` `len``(arr)` `print``(recamanArray(arr, n))` `# This code is contributed``# by mohit kumar`

## C#

 `// C# implementation of the approach``using` `System;``using` `System.Collections.Generic;` `class` `GFG``{` `static` `int` `recamanGenerator(``int` `[]arr, ``int` `n)``{``    ``// First term of the sequence is always 0``    ``arr = 0;` `    ``// Fill remaining terms using recursive``    ``// formula``    ``for` `(``int` `i = 1; i <= n; i++)``    ``{``        ``int` `temp = arr[i - 1] - i;``        ``int` `j;` `        ``for` `(j = 0; j < i; j++)``        ``{` `            ``// If arr[i-1] - i is negative or``            ``// already exists``            ``if` `((arr[j] == temp) || temp < 0)``            ``{``                ``temp = arr[i - 1] + i;``                ``break``;``            ``}``        ``}` `        ``arr[i] = temp;``    ``}``    ``return` `0;``}` `// Function that returns minimum changes required``static` `int` `recamanArray(``int` `[]arr, ``int` `n)``{``    ` `    ``// Set to store first n Recaman numbers``    ``HashSet<``int``> s=``new` `HashSet<``int``>();` `    ``// Generate and store``    ``// first n Recaman numbers``    ``int``[] recaman=``new` `int``[n+1];``    ``recamanGenerator(recaman, n);` `    ``// Insert first n Recaman numbers to set``    ``for` `(``int` `i = 0; i < n; i++)``        ``s.Add(recaman[i]);` `    ``for` `(``int` `i = 0; i < n; i++)``    ``{``        ``// If current element of the array``        ``// is present in the set``        ``if` `(s.Contains(arr[i]))``            ``s.Remove(arr[i]);``    ``}` `    ``// Return the remaining number of``    ``// elements in the set``    ``return` `s.Count;``}` `// Driver code``static` `void` `Main()``{` `    ``int` `[]arr = { 7, 11, 20, 4, 2, 1, 8, 6 };``    ``int` `n = arr.Length;` `    ``Console.Write( recamanArray(arr, n));``}``}` `// This code is contributed by mits`

## Javascript

 ``
Output:
`3`

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