First element that appears even number of times in an array

Given an array, find the first element that appears even number of times in the array. It returns the element if exists otherwise returns 0.

Examples:

Input : arr[] = {1, 5, 4, 7, 4, 1, 5, 7, 1, 5};
Output : 4
Explanation, 4 is the first element that appears even number of times.

Input : arr[] = {2, 4, 6, 8, 1, 6};
Output : 6
Explanation, 6 is the first element that appears even number of times

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

A Simple solution is to consider every element one by one. For every element, start counting the frequency, the first element whose count is even gives the result. Time complexity of this solution is O(n).

An Efficient solution can solve this problem using hash map having O(n) time and O(n) extra space as:

For each element a[i] of the array, do the following:

• If a[i] is not present in the hash map, insert it in hash map as pair (a[i], false).
• If a[i] is present in the hash map and value is true, toggle it to false.
• If a[i] is present in the hash map and value is false, toggle it to true.
• To find the first element appearing even number of times, traverse the array again and find a[i] with value true and return.

C++

 `// C++ code to find the first element ` `// that appears even number times ` `#include ` `using` `namespace` `std; ` `int` `firstEven(``int` `arr[], ``int` `n) ` `{ ` ` `  `    ``unordered_map<``int``, ``bool``> map1; ` ` `  `    ``for` `(``int` `i = 0; i < n; i++) ` `    ``{ ` ` `  `        ``// first time occurred ` `        ``if` `(map1.find(arr[i]) == map1.end()) ` `            ``map1.insert(pair <``int``,``bool``> (arr[i],``false``)); ` ` `  `        ``// toggle for repeated occurrence ` `        ``else` `        ``{ ` `            ``bool` `val = map1.find(arr[i])->second; ` `            ``if` `(val == ``true``) ` `                ``map1.find(arr[i])->second = ``false``; ` `            ``else` `                ``map1.find(arr[i])->second = ``true``; ` `        ``} ` `    ``} ` ` `  `    ``int` `j = 0; ` `    ``for` `(j = 0; j < n; j++) ` `    ``{ ` ` `  `        ``// first integer with true value ` `        ``if` `(map1.find(arr[j])->second == ``true``) ` `            ``break``; ` `    ``} ` ` `  `    ``return` `arr[j]; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``int` `arr[] = { 2, 4, 6, 8, 1, 6 }; ` `    ``cout << firstEven(arr, 6); ` `    ``return` `0; ` `} `

Java

 `// JAVA code to find the first element ` `// that appears even number times ` `import` `java.util.*; ` `class` `GFG { ` `    ``public` `static` `int` `firstEven(``int` `arr[], ``int` `n) ` `    ``{ ` ` `  `        ``HashMap map =  ` `                 ``new` `HashMap(); ` ` `  `        ``for` `(``int` `i = ``0``; i < n; i++) { ` ` `  `            ``// first time occurred ` `            ``if` `(map.get(arr[i]) == ``null``)  ` `                ``map.put(arr[i], ``false``); ` `             `  `            ``// toggle for repeated occurrence ` `            ``else` `{ ` `                ``boolean` `val = map.get(arr[i]); ` `                ``if` `(val == ``true``) ` `                    ``map.put(arr[i], ``false``); ` `                ``else` `                    ``map.put(arr[i], ``true``); ` `            ``} ` `        ``} ` ` `  `        ``int` `j = ``0``; ` `        ``for` `(j = ``0``; j < n; j++) { ` ` `  `            ``// first integer with true value ` `            ``if` `(map.get(arr[j]) == ``true``) ` `                ``break``; ` `        ``} ` ` `  `        ``return` `arr[j]; ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``int` `arr[] = { ``2``, ``4``, ``6``, ``8``, ``1``, ``6` `}; ` `        ``int` `n = arr.length; ` `        ``System.out.println(firstEven(arr, n)); ` `    ``} ` `} `

Python3

 `# Python3 code to find the first element  ` `# that appears even number times  ` `def` `firstEven(arr, n):  ` ` `  `    ``map1 ``=` `{}  ` `    ``for` `i ``in` `range``(``0``, n):  ` `     `  `        ``# first time occurred  ` `        ``if` `arr[i] ``not` `in` `map1:  ` `            ``map1[arr[i]] ``=` `False` ` `  `        ``# toggle for repeated occurrence  ` `        ``else``: ` `            ``map1[arr[i]] ``=` `not` `map1[arr[i]] ` ` `  `    ``for` `j ``in` `range``(``0``, n):  ` `     `  `        ``# first integer with true value  ` `        ``if` `map1[arr[j]] ``=``=` `True``:  ` `            ``break` `     `  `    ``return` `arr[j]  ` ` `  `# Driver code  ` `if` `__name__ ``=``=` `"__main__"``: ` ` `  `    ``arr ``=` `[``2``, ``4``, ``6``, ``8``, ``1``, ``6``]  ` `    ``print``(firstEven(arr, ``6``)) ` ` `  `# This code is contributed  ` `# by Rituraj Jain `

C#

 `// C# code to find the first element ` `// that appears even number times ` `using` `System; ` `using` `System.Collections.Generic; ` ` `  `class` `GFG  ` `{ ` `    ``static` `int` `firstEven(``int` `[]arr,  ` `                         ``int` `n) ` `    ``{ ` `        ``var` `map = ``new` `Dictionary<``int``,  ` `                                 ``string``>(); ` `         `  `        ``var` `hash = ``new` `HashSet<``int``>(arr); ` `         `  `        ``foreach` `(``int` `a ``in` `hash) ` `            ``map.Add(a, ``"null"``); ` `             `  `        ``for` `(``int` `i = 0; i < n; i++)  ` `        ``{ ` ` `  `            ``// first time occurred ` `            ``if` `(map[arr[i]].Equals(``"null"``)) ` `                ``map[arr[i]] = ``"false"``; ` `             `  `            ``// toggle for repeated ` `            ``// occurrence ` `            ``else`  `            ``{ ` `                ``string` `val = map[arr[i]]; ` `                ``if` `(val.Equals(``"true"``)) ` `                    ``map[arr[i]] = ``"false"``; ` `                ``else` `                    ``map[arr[i]] = ``"true"``; ` `            ``} ` `        ``} ` ` `  `        ``int` `j = 0; ` `        ``for` `(j = 0; j < n; j++) ` `        ``{ ` ` `  `            ``// first integer with ` `            ``// true value ` `            ``if` `(map[arr[j]].Equals(``"true"``)) ` `                ``break``; ` `        ``} ` `        ``return` `arr[j]; ` `    ``} ` ` `  `    ``// Driver code ` `    ``static` `void` `Main() ` `    ``{ ` `        ``int` `[]arr = ``new` `int``[]{ 2, 4, 6, ` `                               ``8, 1, 6 }; ` `        ``int` `n = arr.Length; ` `        ``Console.Write(firstEven(arr, n)); ` `    ``} ` `} ` ` `  `// This code is contributed by  ` `// Manish Shaw(manishshaw1) `

Output:

```6
```

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

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Improved By : manishshaw1, rituraj_jain

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