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 ?

Contributed by Ali Ahmet Bingül

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