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.
1 <= nums.length <= 3 * 10 ^4-2 ^31 <= nums[i] <= 2 ^31 - 1Each element in nums appears exactly three times except for one element which appears once .