LeetCode 136 Single Number - Problem Statement

Every element in the array appears exactly twice, except for one element that appears once. Find that single element.

With nums = [2,2,1]:

• 2 appears twice, 1 appears once.
• Answer: $1$.

With nums = [4,1,2,1,2]:

• 1 appears twice, 2 appears twice, 4 appears once.
• Answer: $4$.

Must use $O(1)$ extra space.

Constraints: $1 \le$ nums.length $\le 3 \times 10^4$.