# Find single in an array of 2n+1 integer elements

Given an array with 2n+1 integers, n elements appear twice in arbitrary places in the array and a single integer appears only once somewhere inside. Find the lonely integer with O(n) operations and O(1) extra memory.**Examples :**

Input : { 1, 1, 2, 2, 3, 3, 4, 4, 5} Output : 5 Input : { 7, 9, 6, 8, 3, 7, 8, 6, 9} Output : 3

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The idea is to do XOR of all elements. XOR of all elements gives us the result. The idea is based on below XOR properties.

- XOR of a number with itself is 0.
- XOR of a number with 0 is the number.

## C++

`// CPP program to find only` `// element in an array where` `// every element appears twice.` `#include <bits/stdc++.h>` `using` `namespace` `std;` `// Find non repeating` `// number in an array` `int` `findNonRepeating(` `int` `arr[],` ` ` `int` `n)` `{` ` ` `int` `res = 0;` ` ` `for` `(` `int` `i = 0; i < n; i++)` ` ` `res = res ^ arr[i];` ` ` `return` `res;` `}` `// Driver Code` `int` `main()` `{` ` ` `int` `arr[] = { 3, 8, 3, 2, 2, 1, 1 };` ` ` `int` `n = ` `sizeof` `(arr) / ` `sizeof` `(arr[0]);` ` ` `cout << findNonRepeating(arr, n);` ` ` `return` `0;` `}` |

## Java

`// Java program to find only element` `// in an array where every element` `// appears twice.` `import` `java.io.*;` `class` `GFG` `{` `// Find non repeating` `// number in an array` `static` `int` `findNonRepeating(` `int` `[]arr,` ` ` `int` `n)` `{` ` ` `int` `res = ` `0` `;` ` ` `for` `(` `int` `i = ` `0` `; i < n; i++)` ` ` `res = res ^ arr[i];` ` ` `return` `res;` `}` `// Driver Code` `static` `public` `void` `main (String[] args)` `{` `int` `[]arr = {` `3` `, ` `8` `, ` `3` `, ` `2` `, ` `2` `, ` `1` `, ` `1` `};` `int` `n = arr.length;` `System.out.println(findNonRepeating(arr, n));` `}` `}` `// This code is contributed by vt_m.` |

## Python

`# Find non repeating` `# number in an array` `def` `findNonRepeating(a, n):` ` ` `res ` `=` `0` ` ` ` ` `# XOR of all numbers` ` ` `for` `i ` `in` `range` `(n):` ` ` `res ^` `=` `a[i]` ` ` `return` `res` ` ` `# Driver code` `a ` `=` `[ ` `3` `, ` `8` `, ` `3` `, ` `2` `, ` `2` `, ` `1` `, ` `1` `]` `n ` `=` `len` `(a)` `print` `findNonRepeating(a, n)` `# This code is contributed` `# by 'Striver'.` |

## C#

`// C# program to find only element` `// in an array where every element` `// appears twice.` `using` `System;` `class` `GFG` `{` `// Find non repeating number in an array` `static` `int` `findNonRepeating(` `int` `[]arr,` ` ` `int` `n)` `{` ` ` `int` `res = 0;` ` ` `for` `(` `int` `i = 0; i < n; i++)` ` ` `res = res ^ arr[i];` ` ` `return` `res;` `}` `// Driver Code` `static` `public` `void` `Main (String []args)` `{` `int` `[]arr = {3, 8, 3, 2, 2, 1, 1};` `int` `n = arr.Length;` `Console.WriteLine(findNonRepeating(arr,` ` ` `n));` `}` `}` `// This code is contributed by vt_m.` |

## PHP

`<?php` `// PHP program to find only` `// element in an array where` `// every element appears twice.` `// Find non repeating number` `// in an array` `function` `findNonRepeating(` `$arr` `, ` `$n` `)` `{` ` ` `$res` `= 0;` ` ` `for` `(` `$i` `= 0; ` `$i` `< ` `$n` `; ` `$i` `++)` ` ` `$res` `= ` `$res` `^ ` `$arr` `[` `$i` `];` ` ` `return` `$res` `;` `}` `// Driver Code` `$arr` `= ` `array` `( 3, 8, 3, 2, 2, 1, 1 );` `$n` `= sizeof(` `$arr` `);` `echo` `(findNonRepeating(` `$arr` `, ` `$n` `));` `// This code is contributed by Ajit.` `?>` |

## Javascript

`<script>` `// Javascript program to find only` `// element in an array where` `// every element appears twice.` `// Find non repeating` `// number in an array` `function` `findNonRepeating(arr, n)` `{` ` ` `let res = 0;` ` ` `for` `(let i = 0; i < n; i++)` ` ` `res = res ^ arr[i];` ` ` `return` `res;` `}` `// Driver Code` ` ` `let arr = [ 3, 8, 3, 2, 2, 1, 1 ];` ` ` `let n = arr.length;` ` ` `document.write(findNonRepeating(arr, n));` `</script>` |

**Output :**

8

Time Complexity: O(n).

Space Complexity: O(1).

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