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# Elements that occurred only once in the array

• Difficulty Level : Easy
• Last Updated : 20 May, 2021

Given an array arr that has numbers appearing twice or once. The task is to identify numbers that occur only once in the array.

Note: Duplicates appear side by side every time. There might be a few numbers that can occur at one time and just assume this is a right rotating array (just say an array can rotate k times towards right). The order of the elements in the output doesn’t matter.

Examples:

```Input: arr[] = { 7, 7, 8, 8, 9, 1, 1, 4, 2, 2 }
Output: 9 4

Input: arr[] = {-9, -8, 4, 4, 5, 5, -1}
Output: -9 -8 -1```

Method-1: Using Sorting.

• Sort the array.
• Check for each element at index i (except the first and last element), if
`arr[i] != arr[i-1] && arr [i] != arr[i+1]`
• For the first element, check if arr != arr.
• For the last element, check if arr[n-1] != arr[n-2].

Below is the implementation of the above approach:

## C++

 `// C++ implementation of above approach``#include ``using` `namespace` `std;` `// Function to find the elements that``// appeared only once in the array``void` `occurredOnce(``int` `arr[], ``int` `n)``{``    ``// Sort the array``    ``sort(arr, arr + n);` `    ``// Check for first element``    ``if` `(arr != arr)``        ``cout << arr << ``" "``;` `    ``// Check for all the elements if it is different``    ``// its adjacent elements``    ``for` `(``int` `i = 1; i < n - 1; i++)``        ``if` `(arr[i] != arr[i + 1] && arr[i] != arr[i - 1])``            ``cout << arr[i] << ``" "``;` `    ``// Check for the last element``    ``if` `(arr[n - 2] != arr[n - 1])``        ``cout << arr[n - 1] << ``" "``;``}` `// Driver code``int` `main()``{` `    ``int` `arr[] = { 7, 7, 8, 8, 9, 1, 1, 4, 2, 2 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);` `    ``occurredOnce(arr, n);` `    ``return` `0;``}`

## Java

 `// Java implementation``// of above approach``import` `java.util.*;` `class` `GFG``{` `// Function to find the elements that``// appeared only once in the array``static` `void` `occurredOnce(``int` `arr[], ``int` `n)``{``    ``// Sort the array``    ``Arrays.sort(arr);` `    ``// Check for first element``    ``if` `(arr[``0``] != arr[``1``])``        ``System.out.println(arr[``0``] + ``" "``);` `    ``// Check for all the elements``    ``// if it is different``    ``// its adjacent elements``    ``for` `(``int` `i = ``1``; i < n - ``1``; i++)``        ``if` `(arr[i] != arr[i + ``1``] &&``            ``arr[i] != arr[i - ``1``])``            ``System.out.print(arr[i] + ``" "``);` `    ``// Check for the last element``    ``if` `(arr[n - ``2``] != arr[n - ``1``])``        ``System.out.print(arr[n - ``1``] + ``" "``);``}` `// Driver code``public` `static` `void` `main(String args[])``{``    ``int` `arr[] = {``7``, ``7``, ``8``, ``8``, ``9``,``                 ``1``, ``1``, ``4``, ``2``, ``2``};``    ``int` `n = arr.length;` `    ``occurredOnce(arr, n);``}``}` `// This code is contributed``// by Arnab Kundu`

## Python3

 `# Python 3 implementation``# of above approach` `# Function to find the elements``# that appeared only once in``# the array``def` `occurredOnce(arr, n):``    ` `    ``# Sort the array``    ``arr.sort()` `    ``# Check for first element``    ``if` `arr[``0``] !``=` `arr[``1``]:``        ``print``(arr[``0``], end ``=` `" "``)` `    ``# Check for all the elements``    ``# if it is different its``    ``# adjacent elements``    ``for` `i ``in` `range``(``1``, n ``-` `1``):``        ``if` `(arr[i] !``=` `arr[i ``+` `1``] ``and``            ``arr[i] !``=` `arr[i ``-` `1``]):``            ``print``( arr[i], end ``=` `" "``)` `    ``# Check for the last element``    ``if` `arr[n ``-` `2``] !``=` `arr[n ``-` `1``]:``        ``print``(arr[n ``-` `1``], end ``=` `" "``)` `# Driver code``if` `__name__ ``=``=` `"__main__"``:` `    ``arr ``=` `[ ``7``, ``7``, ``8``, ``8``, ``9``,``            ``1``, ``1``, ``4``, ``2``, ``2` `]``    ``n ``=` `len``(arr)``    ``occurredOnce(arr, n)` `# This code is contributed``# by ChitraNayal`

## C#

 `// C# implementation``// of above approach``using` `System;` `class` `GFG``{` `// Function to find the elements that``// appeared only once in the array``static` `void` `occurredOnce(``int``[] arr, ``int` `n)``{``    ``// Sort the array``    ``Array.Sort(arr);` `    ``// Check for first element``    ``if` `(arr != arr)``        ``Console.Write(arr + ``" "``);` `    ``// Check for all the elements``    ``// if it is different``    ``// its adjacent elements``    ``for` `(``int` `i = 1; i < n - 1; i++)``        ``if` `(arr[i] != arr[i + 1] &&``            ``arr[i] != arr[i - 1])``            ``Console.Write(arr[i] + ``" "``);` `    ``// Check for the last element``    ``if` `(arr[n - 2] != arr[n - 1])``        ``Console.Write(arr[n - 1] + ``" "``);``}` `// Driver code``public` `static` `void` `Main()``{``    ``int``[] arr = {7, 7, 8, 8, 9,``                ``1, 1, 4, 2, 2};``    ``int` `n = arr.Length;` `    ``occurredOnce(arr, n);``}``}` `// This code is contributed``// by ChitraNayal`

## PHP

 ``

## Javascript

 ``
Output
`4 9 `

Time Complexity: O(Nlogn)
Space Complexity: O(1)

Method-2: (Using Hashing): In C++, unordered_map can be used for hashing.

• Traverse the array.
• Store each element with its occurrence in the unordered_map.
• Traverse the unordered_map and print all the elements with occurrence 1.

Below is the implementation of the above approach:

## C++

 `// C++ implementation to find elements``// that appeared only once``#include ``using` `namespace` `std;` `// Function to find the elements that``// appeared only once in the array``void` `occurredOnce(``int` `arr[], ``int` `n)``{``    ``unordered_map<``int``, ``int``> mp;` `    ``// Store all the elements in the map with``    ``// their occurrence``    ``for` `(``int` `i = 0; i < n; i++)``        ``mp[arr[i]]++;` `    ``// Traverse the map and print all the``    ``// elements with occurrence 1``    ``for` `(``auto` `it = mp.begin(); it != mp.end(); it++)``        ``if` `(it->second == 1)``            ``cout << it->first << ``" "``;``}` `// Driver code``int` `main()``{` `    ``int` `arr[] = { 7, 7, 8, 8, 9, 1, 1, 4, 2, 2 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);` `    ``occurredOnce(arr, n);` `    ``return` `0;``}`

## Java

 `// Java implementation to find elements``// that appeared only once``import` `java.util.*;``import` `java.io.*;` `class` `GFG``{``    ``// Function to find the elements that``    ``// appeared only once in the array``    ``static` `void` `occurredOnce(``int``[] arr, ``int` `n)``    ``{``            ``HashMap mp = ``new` `HashMap<>();``            ` `            ``// Store all the elements in the map with``            ``// their occurrence``            ``for` `(``int` `i = ``0``; i < n; i++)``            ``{``                ``if` `(mp.containsKey(arr[i]))``                    ``mp.put(arr[i], ``1` `+ mp.get(arr[i]));``                ``else``                    ``mp.put(arr[i], ``1``);``            ``}` `            ``// Traverse the map and print all the``            ``// elements with occurrence 1``            ``for` `(Map.Entry entry : mp.entrySet())``            ``{``                ``if` `(Integer.parseInt(String.valueOf(entry.getValue())) == ``1``)``                    ``System.out.print(entry.getKey() + ``" "``);``            ``}``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String args[])``    ``{``            ``int``[] arr = { ``7``, ``7``, ``8``, ``8``, ``9``, ``1``, ``1``, ``4``, ``2``, ``2` `};``            ``int` `n = arr.length;` `            ``occurredOnce(arr, n);``    ``}``}` `// This code is contributed by rachana soma`

## Python3

 `# Python3 implementation to find elements``# that appeared only once``import` `math as mt` `# Function to find the elements that``# appeared only once in the array``def` `occurredOnce(arr, n):` `    ``mp ``=` `dict``()` `    ``# Store all the elements in the``    ``# map with their occurrence``    ``for` `i ``in` `range``(n):``        ``if` `arr[i] ``in` `mp.keys():``            ``mp[arr[i]] ``+``=` `1``        ``else``:``            ``mp[arr[i]] ``=` `1` `    ``# Traverse the map and print all``    ``# the elements with occurrence 1``    ``for` `it ``in` `mp:``        ``if` `mp[it] ``=``=` `1``:``            ``print``(it, end ``=` `" "``)` `# Driver code``arr ``=` `[``7``, ``7``, ``8``, ``8``, ``9``, ``1``, ``1``, ``4``, ``2``, ``2``]``n ``=` `len``(arr)` `occurredOnce(arr, n)` `# This code is contributed by``# Mohit Kumar 29`

## C#

 `// C# implementation to find elements``// that appeared only once``using` `System;``using` `System.Collections.Generic;``class` `GFG``{``    ``// Function to find the elements that``    ``// appeared only once in the array``    ``static` `void` `occurredOnce(``int``[] arr, ``int` `n)``    ``{``      ``Dictionary<``int``, ``int``> mp = ``new` `Dictionary<``int``, ``int``>();` `      ``// Store all the elements in the map with``      ``// their occurrence``      ``for` `(``int` `i = 0; i < n; i++)``      ``{``        ``if` `(mp.ContainsKey(arr[i]))``          ``mp[arr[i]] = 1 + mp[arr[i]];``        ``else``          ``mp.Add(arr[i], 1);``      ``}` `      ``// Traverse the map and print all the``      ``// elements with occurrence 1``      ``foreach``(KeyValuePair<``int``, ``int``> entry ``in` `mp)``      ``{``        ``if` `(Int32.Parse(String.Join(``""``, entry.Value)) == 1)``          ``Console.Write(entry.Key + ``" "``);``      ``}``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main(String []args)``    ``{``      ``int``[] arr = { 7, 7, 8, 8, 9, 1, 1, 4, 2, 2 };``      ``int` `n = arr.Length;``      ``occurredOnce(arr, n);``    ``}``}` `// This code is contributed by shikhasingrajput`

## Javascript

 ``
Output
`4 9 `

Time Complexity: O(N)
Space Complexity: O(N)

Method-3: Using given assumptions.
It is given that an array can be rotated any time and duplicates will appear side by side every time. So, after rotating, the first and last elements will appear side by side.

• Check if the first and last elements are equal. If yes, then start traversing the elements between them.
• Check if the current element is equal to the element in the immediate previous index. If yes, check the same for the next element.
• If not, print the current element.

## C++

 `// C++ implementation to find elements``// that appeared only once``#include ``using` `namespace` `std;` `// Function to find the elements that``// appeared only once in the array``void` `occurredOnce(``int` `arr[], ``int` `n)``{``    ``int` `i = 1, len = n;` `    ``// Check if the first and last element is equal``    ``// If yes, remove those elements``    ``if` `(arr == arr[len - 1]) {``        ``i = 2;``        ``len--;``    ``}` `    ``// Start traversing the remaining elements``    ``for` `(; i < n; i++)` `        ``// Check if current element is equal to``        ``// the element at immediate previous index``        ``// If yes, check the same for next element``        ``if` `(arr[i] == arr[i - 1])``            ``i++;` `        ``// Else print the current element``        ``else``            ``cout << arr[i - 1] << ``" "``;` `    ``// Check for the last element``    ``if` `(arr[n - 1] != arr && arr[n - 1] != arr[n - 2])``        ``cout << arr[n - 1];``}` `// Driver code``int` `main()``{` `    ``int` `arr[] = { 7, 7, 8, 8, 9, 1, 1, 4, 2, 2 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);` `    ``occurredOnce(arr, n);` `    ``return` `0;``}`

## Java

 `// Java implementation to find``// elements that appeared only once``class` `GFG``{``// Function to find the elements that``// appeared only once in the array``static` `void` `occurredOnce(``int` `arr[], ``int` `n)``{``    ``int` `i = ``1``, len = n;` `    ``// Check if the first and last``    ``// element is equal. If yes,``    ``// remove those elements``    ``if` `(arr[``0``] == arr[len - ``1``])``    ``{``        ``i = ``2``;``        ``len--;``    ``}` `    ``// Start traversing the``    ``// remaining elements``    ``for` `(; i < n; i++)` `        ``// Check if current element is``        ``// equal to the element at``        ``// immediate previous index``        ``// If yes, check the same``        ``// for next element``        ``if` `(arr[i] == arr[i - ``1``])``            ``i++;` `        ``// Else print the current element``        ``else``            ``System.out.print(arr[i - ``1``] + ``" "``);` `    ``// Check for the last element``    ``if` `(arr[n - ``1``] != arr[``0``] &&``        ``arr[n - ``1``] != arr[n - ``2``])``        ``System.out.print(arr[n - ``1``]);``}` `// Driver code``public` `static` `void` `main(String args[])``{``    ``int` `arr[] = {``7``, ``7``, ``8``, ``8``, ``9``,``                 ``1``, ``1``, ``4``, ``2``, ``2``};``    ``int` `n = arr.length;` `    ``occurredOnce(arr, n);``}``}` `// This code is contributed``// by Arnab Kundu`

## Python3

 `# Python 3 implementation to find``# elements that appeared only once` `# Function to find the elements that``# appeared only once in the array``def` `occurredOnce(arr, n):``    ``i ``=` `1``    ``len` `=` `n` `    ``# Check if the first and``    ``# last element is equal``    ``# If yes, remove those elements``    ``if` `arr[``0``] ``=``=` `arr[``len` `-` `1``]:``        ``i ``=` `2``        ``len` `-``=` `1` `    ``# Start traversing the``    ``# remaining elements``    ``while` `i < n:` `        ``# Check if current element is``        ``# equal to the element at``        ``# immediate previous index``        ``# If yes, check the same for``        ``# next element``        ``if` `arr[i] ``=``=` `arr[i ``-` `1``]:``            ``i ``+``=` `1` `        ``# Else print the current element``        ``else``:``            ``print``(arr[i ``-` `1``], end ``=` `" "``)``            ` `        ``i ``+``=` `1` `    ``# Check for the last element``    ``if` `(arr[n ``-` `1``] !``=` `arr[``0``] ``and``        ``arr[n ``-` `1``] !``=` `arr[n ``-` `2``]):``        ``print``(arr[n ``-` `1``])` `# Driver code``if` `__name__ ``=``=` `"__main__"``:``    ``arr ``=` `[ ``7``, ``7``, ``8``, ``8``, ``9``, ``1``, ``1``, ``4``, ``2``, ``2` `]``    ``n ``=` `len``(arr)` `    ``occurredOnce(arr, n)` `# This code is contributed``# by ChitraNayal`

## C#

 `// C# implementation to find``// elements that appeared only once``using` `System;` `class` `GFG``{``// Function to find the elements that``// appeared only once in the array``static` `void` `occurredOnce(``int``[] arr, ``int` `n)``{``    ``int` `i = 1, len = n;` `    ``// Check if the first and last``    ``// element is equal. If yes,``    ``// remove those elements``    ``if` `(arr == arr[len - 1])``    ``{``        ``i = 2;``        ``len--;``    ``}` `    ``// Start traversing the``    ``// remaining elements``    ``for` `(; i < n; i++)` `        ``// Check if current element is``        ``// equal to the element at``        ``// immediate previous index``        ``// If yes, check the same``        ``// for next element``        ``if` `(arr[i] == arr[i - 1])``            ``i++;` `        ``// Else print the current element``        ``else``            ``Console.Write(arr[i - 1] + ``" "``);` `    ``// Check for the last element``    ``if` `(arr[n - 1] != arr &&``        ``arr[n - 1] != arr[n - 2])``        ``Console.Write(arr[n - 1]);``}` `// Driver code``public` `static` `void` `Main()``{``    ``int``[] arr = {7, 7, 8, 8, 9,``                ``1, 1, 4, 2, 2};``    ``int` `n = arr.Length;` `    ``occurredOnce(arr, n);``}``}` `// This code is contributed``// by ChitraNayal`

## PHP

 ``

## Javascript

 ``
Output
`9 4 `

Time Complexity: O(N)
Space Complexity: O(1)

#### Method #4:Using built-in Python functions:

• Count the frequencies of every element using the Counter function
• Traverse the frequency array and print all the elements with occurrence 1.

Below is the implementation

## Python3

 `# Python3 implementation to find elements``# that appeared only once``from` `collections ``import` `Counter` `# Function to find the elements that``# appeared only once in the array``def` `occurredOnce(arr, n):` `    ``#counting frequency of every element using Counter``    ``mp``=``Counter(arr)``    ``# Traverse the map and print all``    ``# the elements with occurrence 1``    ``for` `it ``in` `mp:``        ``if` `mp[it] ``=``=` `1``:``            ``print``(it, end ``=` `" "``)` `# Driver code``arr ``=` `[``7``, ``7``, ``8``, ``8``, ``9``, ``1``, ``1``, ``4``, ``2``, ``2``]``n ``=` `len``(arr)` `occurredOnce(arr, n)` `# This code is contributed by vikkycirus`
Output
`9 4 `

Time Complexity: O(n)

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