136\. Single Number

Easy

Given a **non-empty** array of integers `nums`, every element appears _twice_ except for one. Find that single one.

You must implement a solution with a linear runtime complexity and use only constant extra space.

**Example 1:**

**Input:** nums = [2,2,1]

**Output:** 1 

**Example 2:**

**Input:** nums = [4,1,2,1,2]

**Output:** 4 

**Example 3:**

**Input:** nums = [1]

**Output:** 1 

**Constraints:**

*   <code>1 <= nums.length <= 3 * 10<sup>4</sup></code>
*   <code>-3 * 10<sup>4</sup> <= nums[i] <= 3 * 10<sup>4</sup></code>
*   Each element in the array appears twice except for one element which appears only once.