Single Number II

Given an integer array `nums` where every element appears **three times** except for one, which appears **exactly once**. _Find the single element and return it_. You must implement a solution with a linear runtime complexity and use only constant extra space.   **Example 1:** **Input:** nums = [2,2,3,2] **Output:** 3 ``` **Example 2:** **Input:** nums = [0,1,0,1,0,1,99] **Output:** 99 ```   **Constraints:** - `1 <= nums.length <= 3 * 104` - `-231 <= nums[i] <= 231 - 1` - Each element in `nums` appears exactly **three times** except for one element which appears **once**.