Given a non-empty array arr, in which exactly two elements appear once, and all the other elements appear twice, return the two elements that appeared only once.

Note: The result can be returned in any order. The solution should use only constant extra space.

Constraints:

• $2 \leq$ arr.length $\leq 10^3$

• $−2^{31} \leq$ arr[i] $\leq 2^{31}-1$