# Non-Repeating Element

Find the first non-repeating element in a given array of integers.

Examples:

```Input : -1 2 -1 3 2
Output : 3
Explanation : The first number that does not
repeat is : 3

Input : 9 4 9 6 7 4
Output : 6
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

A Simple Solution is to use two loops. The outer loop picks elements one by one and inner loop checks if the element is present more than once or not.

## C++

 `// Simple CPP program to find first non- ` `// repeating element. ` `#include ` `using` `namespace` `std; ` ` `  `int` `firstNonRepeating(``int` `arr[], ``int` `n) ` `{ ` `    ``for` `(``int` `i = 0; i < n; i++) { ` `        ``int` `j; ` `        ``for` `(j = 0; j < n; j++) ` `            ``if` `(i != j && arr[i] == arr[j]) ` `                ``break``; ` `        ``if` `(j == n) ` `            ``return` `arr[i]; ` `    ``} ` `    ``return` `-1; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``int` `arr[] = { 9, 4, 9, 6, 7, 4 }; ` `    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr); ` `    ``cout << firstNonRepeating(arr, n); ` `    ``return` `0; ` `} `

## Java

 `// Java program to find first non-repeating ` `// element. ` `class` `GFG { ` ` `  `    ``static` `int` `firstNonRepeating(``int` `arr[], ``int` `n) ` `    ``{ ` `        ``for` `(``int` `i = ``0``; i < n; i++) { ` `            ``int` `j; ` `            ``for` `(j = ``0``; j < n; j++) ` `                ``if` `(i != j && arr[i] == arr[j]) ` `                    ``break``; ` `            ``if` `(j == n) ` `                ``return` `arr[i]; ` `        ``} ` ` `  `        ``return` `-``1``; ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` ` `  `        ``int` `arr[] = { ``9``, ``4``, ``9``, ``6``, ``7``, ``4` `}; ` `        ``int` `n = arr.length; ` ` `  `        ``System.out.print(firstNonRepeating(arr, n)); ` `    ``} ` `} ` ` `  `// This code is contributed by Anant Agarwal. `

## Python3

 `# Python3 program to find first  ` `# non-repeating element. ` ` `  `def` `firstNonRepeating(arr, n): ` ` `  `    ``for` `i ``in` `range``(n): ` `        ``j ``=` `0` `        ``while``(j < n): ` `            ``if` `(i !``=` `j ``and` `arr[i] ``=``=` `arr[j]): ` `                ``break` `            ``j ``+``=` `1` `        ``if` `(j ``=``=` `n): ` `            ``return` `arr[i] ` `     `  `    ``return` `-``1` `     `  `# Driver code ` `arr ``=` `[ ``9``, ``4``, ``9``, ``6``, ``7``, ``4` `] ` `n ``=` `len``(arr) ` `print``(firstNonRepeating(arr, n)) ` ` `  `# This code is contributed by Anant Agarwal. `

## C#

 `// C# program to find first non- ` `// repeating element. ` `using` `System; ` ` `  `class` `GFG { ` `    ``static` `int` `firstNonRepeating(``int``[] arr, ``int` `n) ` `    ``{ ` `        ``for` `(``int` `i = 0; i < n; i++) { ` `            ``int` `j; ` `            ``for` `(j = 0; j < n; j++) ` `                ``if` `(i != j && arr[i] == arr[j]) ` `                    ``break``; ` `            ``if` `(j == n) ` `                ``return` `arr[i]; ` `        ``} ` `        ``return` `-1; ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `Main() ` `    ``{ ` `        ``int``[] arr = { 9, 4, 9, 6, 7, 4 }; ` `        ``int` `n = arr.Length; ` `        ``Console.Write(firstNonRepeating(arr, n)); ` `    ``} ` `} ` `// This code is contributed by Anant Agarwal. `

## PHP

 ` `

Output:

```6
```

An Efficient Solution is to use hashing.
1) Traverse array and insert elements and their counts in hash table.
2) Traverse array again and print first element with count equals to 1.

## C++

 `// Efficient CPP program to find first non- ` `// repeating element. ` `#include ` `using` `namespace` `std; ` ` `  `int` `firstNonRepeating(``int` `arr[], ``int` `n) ` `{ ` `    ``// Insert all array elements in hash ` `    ``// table ` `    ``unordered_map<``int``, ``int``> mp; ` `    ``for` `(``int` `i = 0; i < n; i++) ` `        ``mp[arr[i]]++; ` ` `  `    ``// Traverse array again and return ` `    ``// first element with count 1. ` `    ``for` `(``int` `i = 0; i < n; i++) ` `        ``if` `(mp[arr[i]] == 1) ` `            ``return` `arr[i]; ` `    ``return` `-1; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``int` `arr[] = { 9, 4, 9, 6, 7, 4 }; ` `    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr); ` `    ``cout << firstNonRepeating(arr, n); ` `    ``return` `0; ` `} `

## Java

 `// Efficient Java program to find first non- ` `// repeating element. ` `import` `java.util.*; ` ` `  `class` `GFG { ` ` `  `    ``static` `int` `firstNonRepeating(``int` `arr[], ``int` `n) ` `    ``{ ` `        ``// Insert all array elements in hash ` `        ``// table ` ` `  `        ``Map m = ``new` `HashMap<>(); ` `        ``for` `(``int` `i = ``0``; i < n; i++) { ` `            ``if` `(m.containsKey(arr[i])) { ` `                ``m.put(arr[i], m.get(arr[i]) + ``1``); ` `            ``} ` `            ``else` `{ ` `                ``m.put(arr[i], ``1``); ` `            ``} ` `        ``} ` `        ``// Traverse array again and return ` `        ``// first element with count 1. ` `        ``for` `(``int` `i = ``0``; i < n; i++) ` `            ``if` `(m.get(arr[i]) == ``1``) ` `                ``return` `arr[i]; ` `        ``return` `-``1``; ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``int` `arr[] = { ``9``, ``4``, ``9``, ``6``, ``7``, ``4` `}; ` `        ``int` `n = arr.length; ` `        ``System.out.println(firstNonRepeating(arr, n)); ` `    ``} ` `} ` ` `  `// This code contributed by Rajput-Ji `

## Python3

 `# Efficient Python3 program to find first  ` `# non-repeating element. ` `from` `collections ``import` `defaultdict ` ` `  `def` `firstNonRepeating(arr, n): ` `    ``mp ``=` `defaultdict(``lambda``:``0``) ` ` `  `    ``# Insert all array elements in hash table  ` `    ``for` `i ``in` `range``(n): ` `        ``mp[arr[i]] ``+``=` `1` ` `  `    ``# Traverse array again and return  ` `    ``# first element with count 1.  ` `    ``for` `i ``in` `range``(n): ` `        ``if` `mp[arr[i]] ``=``=` `1``: ` `            ``return` `arr[i] ` `    ``return` `-``1` ` `  `# Driver Code ` `arr ``=` `[``9``, ``4``, ``9``, ``6``, ``7``, ``4``] ` `n ``=` `len``(arr) ` `print``(firstNonRepeating(arr, n))  ` ` `  `# This code is contributed by Shrikant13 `

## C#

 `// Efficient C# program to find first non- ` `// repeating element. ` `using` `System; ` `using` `System.Collections.Generic; ` ` `  `class` `GFG { ` ` `  `    ``static` `int` `firstNonRepeating(``int``[] arr, ``int` `n) ` `    ``{ ` `        ``// Insert all array elements in hash ` `        ``// table ` ` `  `        ``Dictionary<``int``, ``int``> m = ``new` `Dictionary<``int``, ``int``>(); ` `        ``for` `(``int` `i = 0; i < n; i++) { ` `            ``if` `(m.ContainsKey(arr[i])) { ` `                ``var` `val = m[arr[i]]; ` `                ``m.Remove(arr[i]); ` `                ``m.Add(arr[i], val + 1); ` `            ``} ` `            ``else` `{ ` `                ``m.Add(arr[i], 1); ` `            ``} ` `        ``} ` ` `  `        ``// Traverse array again and return ` `        ``// first element with count 1. ` `        ``for` `(``int` `i = 0; i < n; i++) ` `            ``if` `(m[arr[i]] == 1) ` `                ``return` `arr[i]; ` `        ``return` `-1; ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `Main(String[] args) ` `    ``{ ` `        ``int``[] arr = { 9, 4, 9, 6, 7, 4 }; ` `        ``int` `n = arr.Length; ` `        ``Console.WriteLine(firstNonRepeating(arr, n)); ` `    ``} ` `} ` ` `  `// This code has been contributed by 29AjayKumar `

Output:

```6
```

Time Complexity: O(n)
Auxiliary Space: O(n)

Further Optimization: If array has many duplicates, we can also store index in hash table, using a hash table where value is a pair. Now we only need to traverse keys in hash table (not complete array) to find first non repeating.

Printing all non-repeating elements:

## C++

 `// Efficient CPP program to print all non- ` `// repeating elements. ` `#include ` `using` `namespace` `std; ` ` `  `void` `firstNonRepeating(``int` `arr[], ``int` `n) ` `{ ` `    ``// Insert all array elements in hash ` `    ``// table ` `    ``unordered_map<``int``, ``int``> mp; ` `    ``for` `(``int` `i = 0; i < n; i++) ` `        ``mp[arr[i]]++; ` ` `  `    ``// Traverse through map only and ` `    ``for` `(``auto` `x : mp) ` `        ``if` `(x.second == 1) ` `            ``cout << x.first << ``" "``; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``int` `arr[] = { 9, 4, 9, 6, 7, 4 }; ` `    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr); ` `    ``firstNonRepeating(arr, n); ` `    ``return` `0; ` `} `

## Java

 `// Efficient Java program to print all non- ` `// repeating elements. ` `import` `java.util.*; ` ` `  `class` `GFG { ` ` `  `    ``static` `void` `firstNonRepeating(``int` `arr[], ``int` `n) ` `    ``{ ` `        ``// Insert all array elements in hash ` `        ``// table ` `        ``Map m = ``new` `HashMap<>(); ` `        ``for` `(``int` `i = ``0``; i < n; i++) { ` `            ``if` `(m.containsKey(arr[i])) { ` `                ``m.put(arr[i], m.get(arr[i]) + ``1``); ` `            ``} ` `            ``else` `{ ` `                ``m.put(arr[i], ``1``); ` `            ``} ` `        ``} ` ` `  `        ``// Traverse through map only and ` `        ``// using for-each loop for iteration over Map.entrySet() ` `        ``for` `(Map.Entry x : m.entrySet()) ` `            ``if` `(x.getValue() == ``1``) ` `                ``System.out.print(x.getKey() + ``" "``); ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``int` `arr[] = { ``9``, ``4``, ``9``, ``6``, ``7``, ``4` `}; ` `        ``int` `n = arr.length; ` `        ``firstNonRepeating(arr, n); ` `    ``} ` `} ` ` `  `// This code has been contributed by 29AjayKumar `

## C#

 `// Efficient C# program to print all non- ` `// repeating elements. ` `using` `System; ` `using` `System.Collections.Generic; ` ` `  `class` `GFG { ` ` `  `    ``static` `void` `firstNonRepeating(``int``[] arr, ``int` `n) ` `    ``{ ` `        ``// Insert all array elements in hash ` `        ``// table ` `        ``Dictionary<``int``, ``int``> m = ``new` `Dictionary<``int``, ``int``>(); ` `        ``for` `(``int` `i = 0; i < n; i++) { ` `            ``if` `(m.ContainsKey(arr[i])) { ` `                ``var` `val = m[arr[i]]; ` `                ``m.Remove(arr[i]); ` `                ``m.Add(arr[i], val + 1); ` `            ``} ` `            ``else` `{ ` `                ``m.Add(arr[i], 1); ` `            ``} ` `        ``} ` ` `  `        ``// Traverse through map only and ` `        ``// using for-each loop for iteration over Map.entrySet() ` `        ``foreach``(KeyValuePair<``int``, ``int``> x ``in` `m) ` `        ``{ ` `            ``if` `(x.Value == 1) { ` `                ``Console.Write(x.Key + ``" "``); ` `            ``} ` `        ``} ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `Main(String[] args) ` `    ``{ ` `        ``int``[] arr = { 9, 4, 9, 6, 7, 4 }; ` `        ``int` `n = arr.Length; ` `        ``firstNonRepeating(arr, n); ` `    ``} ` `} ` ` `  `/* This code contributed by PrinciRaj1992 */`

Output:

```7 6
```

My Personal Notes arrow_drop_up Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.