Suppose you have an array of elements (integers), each element in the array occurs twice in the array except for one. How would you find that one element?

Constraints:

`O(n)`

Complexity

`O(1)`

Space (Means no Dictionaries/HashMaps!)

Bonus: Can you do it in one line?

**Solution**

```
/// Lonely Integer Problem
let array = [9, 1, 2, 3, 2, 9 ,1, 7, 7]
/// Find the number that only occurs once in the array.
func lonelyInteger(in array: [Int]) -> Int {
// Use bit manipulation!
return array.reduce(0) { $0 ^ $1 }
}
print(lonelyInteger(in: array)) // 3
```