137. Single Number II 
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Problem
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
numsappears exactly three times except for one element which appears once.