# Convert an array to reduced form | Set 1 (Simple and Hashing)

• Difficulty Level : Easy
• Last Updated : 22 Aug, 2022

Given an array with N distinct elements, convert the given array to a form where all elements are in the range from 0 to N-1. The order of elements is the same, i.e., 0 is placed in the place of the smallest element, 1 is placed for the second smallest element, … N-1 is placed for the largest element.

Examples:

Input:  arr[] = {10, 40, 20}
Output: arr[] = {0, 2, 1}

Input:  arr[] = {5, 10, 40, 30, 20}
Output: arr[] = {0, 1, 4, 3, 2}

Naive Approach:

A simple solution is to first find the minimum element, replace it with 0, consider the remaining array and find the minimum in the remaining array and replace it with 1, and so on.

Time complexity: O(N2)
Auxiliary space: O(N)

Efficient Approach:

The idea is to sort the given array and use an unordered map to store the reduced form of each value of array then update the whole array to its reduced form using values from unordered map.

Follow the below steps to implement the idea:

• Create a temp array and copy the contents of the given array to temp[].
• Sort temp[] in ascending order.
• Create an empty hash table.
• Traverse temp[] from left to right and store mapping of numbers and their values (in converted array) in the hash table.
• Traverse given array and change elements to their positions using a hash table.

Below are implementations of the above idea.

## C++

 `// C++ program to convert an array in reduced``// form``#include ``using` `namespace` `std;` `void` `convert(``int` `arr[], ``int` `n)``{``    ``// Create a temp array and copy contents``    ``// of arr[] to temp``    ``int` `temp[n];``    ``memcpy``(temp, arr, n*``sizeof``(``int``));` `    ``// Sort temp array``    ``sort(temp, temp + n);` `    ``// Create a hash table. Refer``    ``// http://tinyurl.com/zp5wgef``    ``unordered_map<``int``, ``int``> umap;` `    ``// One by one insert elements of sorted``    ``// temp[] and assign them values from 0``    ``// to n-1``    ``int` `val = 0;``    ``for` `(``int` `i = 0; i < n; i++)``        ``umap[temp[i]] = val++;` `    ``// Convert array by taking positions from``    ``// umap``    ``for` `(``int` `i = 0; i < n; i++)``        ``arr[i] = umap[arr[i]];``}` `void` `printArr(``int` `arr[], ``int` `n)``{``    ``for` `(``int` `i=0; i

## Java

 `// Java Program to convert an Array``// to reduced form``import` `java.util.*;` `class` `GFG``{``    ``public` `static` `void` `convert(``int` `arr[], ``int` `n)``    ``{``        ``// Create a temp array and copy contents``        ``// of arr[] to temp``        ``int` `temp[] = arr.clone();` `        ``// Sort temp array``        ``Arrays.sort(temp);` `        ``// Create a hash table.``        ``HashMap umap = ``new` `HashMap<>();` `        ``// One by one insert elements of sorted``        ``// temp[] and assign them values from 0``        ``// to n-1``        ``int` `val = ``0``;``        ``for` `(``int` `i = ``0``; i < n; i++)``            ``umap.put(temp[i], val++);` `        ``// Convert array by taking positions from``        ``// umap``        ``for` `(``int` `i = ``0``; i < n; i++)``            ``arr[i] = umap.get(arr[i]);``    ``}` `    ``public` `static` `void` `printArr(``int` `arr[], ``int` `n)``    ``{``        ``for` `(``int` `i = ``0``; i < n; i++)``            ``System.out.print(arr[i] + ``" "``);``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String[] args)``    ``{` `        ``int` `arr[] = {``10``, ``20``, ``15``, ``12``, ``11``, ``50``};``        ``int` `n = arr.length;` `        ``System.out.println(``"Given Array is "``);``        ``printArr(arr, n);` `        ``convert(arr , n);` `        ``System.out.println(``"\n\nConverted Array is "``);``        ``printArr(arr, n);` `    ``}``}` `// This code is contributed by Abhishek Panwar`

## Python3

 `# Python3 program to convert an array``# in reduced form``def` `convert(arr, n):``    ``# Create a temp array and copy contents``    ``# of arr[] to temp``    ``temp ``=` `[arr[i] ``for` `i ``in` `range` `(n) ]``    ` `    ``# Sort temp array``    ``temp.sort()``    ` `    ``# create a map``    ``umap ``=` `{}``    ` `    ` `    ``# One by one insert elements of sorted``    ``# temp[] and assign them values from 0``    ``# to n-1``    ``val ``=` `0``    ``for` `i ``in` `range` `(n):``        ``umap[temp[i]] ``=` `val``        ``val ``+``=` `1``    ` `    ``# Convert array by taking positions from umap``    ``for` `i ``in` `range` `(n):``        ``arr[i] ``=` `umap[arr[i]]``    ` `def` `printArr(arr, n):``    ``for` `i ``in` `range``(n):``        ``print``(arr[i], end ``=` `" "``)` `# Driver Code``if` `__name__ ``=``=` `"__main__"``:``    ``arr ``=` `[``10``, ``20``, ``15``, ``12``, ``11``, ``50``]``    ``n ``=` `len``(arr)``    ``print``(``"Given Array is "``)``    ``printArr(arr, n)``    ``convert(arr , n)``    ``print``(``"\n\nConverted Array is "``)``    ``printArr(arr, n)` `# This code is contributed by Abhishek Gupta`

## C#

 `// C# Program to convert an Array``// to reduced form``using` `System;``using` `System.Collections.Generic;``using` `System.Linq;` `class` `GFG``{``    ``public` `static` `void` `convert(``int` `[]arr, ``int` `n)``    ``{``        ``// Create a temp array and copy contents``        ``// of []arr to temp``        ``int` `[]temp = ``new` `int``[arr.Length];``        ``Array.Copy(arr, 0, temp, 0, arr.Length);` `        ``// Sort temp array``        ``Array.Sort(temp);` `        ``// Create a hash table.``        ``Dictionary<``int``, ``int``> umap =``            ``new` `Dictionary<``int``, ``int``>();` `        ``// One by one insert elements of sorted``        ``// []temp and assign them values from 0``        ``// to n - 1``        ``int` `val = 0;``        ``for` `(``int` `i = 0; i < n; i++)``            ``if``(umap.ContainsKey(temp[i]))``                ``umap[temp[i]] = val++;``            ``else``                ``umap.Add(temp[i], val++);` `        ``// Convert array by taking positions from``        ``// umap``        ``for` `(``int` `i = 0; i < n; i++)``            ``arr[i] = umap[arr[i]];``    ``}` `    ``public` `static` `void` `printArr(``int` `[]arr, ``int` `n)``    ``{``        ``for` `(``int` `i = 0; i < n; i++)``            ``Console.Write(arr[i] + ``" "``);``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main(String[] args)``    ``{` `        ``int` `[]arr = {10, 20, 15, 12, 11, 50};``        ``int` `n = arr.Length;` `        ``Console.WriteLine(``"Given Array is "``);``        ``printArr(arr, n);` `        ``convert(arr , n);` `        ``Console.WriteLine(``"\n\nConverted Array is "``);``        ``printArr(arr, n);``    ``}``}` `// This code is contributed by PrinciRaj1992`

## Javascript

 ``

Output

```Given Array is
10 20 15 12 11 50

Converted Array is
0 4 3 2 1 5 ```

Time complexity: O(N * log N)
Auxiliary Space: O(N)

Convert an array to reduced form | Set 2 (Using vector of pairs)