# C/C++ Program to Find the Number Occurring Odd Number of Times

Given an array of positive integers. All numbers occur even number of times except one number which occurs odd number of times. Find the number in O(n) time & constant space.

Examples :

```Input : arr = {1, 2, 3, 2, 3, 1, 3}
Output : 3

Input : arr = {5, 7, 2, 7, 5, 2, 5}
Output : 5
```

## C++

 `// C++ program to find the element ` `// occurring odd number of times ` `#include ` `using` `namespace` `std; ` ` `  `// Function to find the element ` `// occurring odd number of times ` `int` `getOddOccurrence(``int` `arr[], ``int` `arr_size) ` `{ ` `    ``for` `(``int` `i = 0; i < arr_size; i++) { ` ` `  `        ``int` `count = 0; ` ` `  `        ``for` `(``int` `j = 0; j < arr_size; j++) { ` `            ``if` `(arr[i] == arr[j]) ` `                ``count++; ` `        ``} ` `        ``if` `(count % 2 != 0) ` `            ``return` `arr[i]; ` `    ``} ` `    ``return` `-1; ` `} ` ` `  `// driver code ` `int` `main() ` `{ ` `    ``int` `arr[] = { 2, 3, 5, 4, 5, 2, ` `                  ``4, 3, 5, 2, 4, 4, 2 }; ` `    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr); ` ` `  `    ``// Function calling ` `    ``cout << getOddOccurrence(arr, n); ` ` `  `    ``return` `0; ` `} `

Output:

```5
```

A Better Solution is to use Hashing. Use array elements as key and their counts as value. Create an empty hash table. One by one traverse the given array elements and store counts. Time complexity of this solution is O(n). But it requires extra space for hashing.

The Best Solution is to do bitwise XOR of all the elements. XOR of all elements gives us odd occurring element. Please note that XOR of two elements is 0 if both elements are same and XOR of a number x with 0 is x.

Below is the implementation of the above approach.

## C++

 `// C++ program to find the element ` `// occurring odd number of times ` `#include ` `using` `namespace` `std; ` ` `  `// Function to find element occurring ` `// odd number of times ` `int` `getOddOccurrence(``int` `ar[], ``int` `ar_size) ` `{ ` `    ``int` `res = 0; ` `    ``for` `(``int` `i = 0; i < ar_size; i++) ` `        ``res = res ^ ar[i]; ` ` `  `    ``return` `res; ` `} ` ` `  `/* Driver function to test above function */` `int` `main() ` `{ ` `    ``int` `ar[] = { 2, 3, 5, 4, 5, 2, 4, 3, 5, 2, 4, 4, 2 }; ` `    ``int` `n = ``sizeof``(ar) / ``sizeof``(ar); ` ` `  `    ``// Function calling ` `    ``cout << getOddOccurrence(ar, n); ` ` `  `    ``return` `0; ` `} `

Output:

```5
```

## C

 `// C program to find the element ` `// occurring odd number of times ` `#include ` ` `  `// Function to find element occurring ` `// odd number of times ` `int` `getOddOccurrence(``int` `ar[], ``int` `ar_size) ` `{ ` `    ``int` `res = 0; ` `    ``for` `(``int` `i = 0; i < ar_size; i++) ` `        ``res = res ^ ar[i]; ` ` `  `    ``return` `res; ` `} ` ` `  `/* Driver function to test above function */` `int` `main() ` `{ ` `    ``int` `ar[] = { 2, 3, 5, 4, 5, 2, 4, 3, 5, 2, 4, 4, 2 }; ` `    ``int` `n = ``sizeof``(ar) / ``sizeof``(ar); ` ` `  `    ``// Function calling ` `    ``printf``(``"%d"``, getOddOccurrence(ar, n)); ` `    ``return` `0; ` `} `

Output:

```5
```

Please refer complete article on Find the Number Occurring Odd Number of Times for more details!

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Improved By : nidhi_biet, shubham_singh

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