You are given an array of non-negative integers where each element occurs exactly twice except one which occurs once.
Find the element occurring once.
Examples:
Input 1:
[1, 1, 2, 2, 3]
Output 1:
3
Explanation 1:
There are two 1s and two 2s whereas only one 3.
Follow Up: Can you do it using constant space and linear time ?