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.

Input: nums = [2,2,3,2]
Output: 3

Input: nums = [0,1,0,1,0,1,99]
Output: 99

• 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.

impl Solution {
    pub fn single_number(nums: Vec<i32>) -> i32 {
        let mut bitcount = [0; 32];
        let mut ret = 0;

        for num in &nums {
            for i in 0..32 {
                if (num >> i) & 1 == 1 {
                    bitcount[i] += 1;
                }
            }
        }

        for i in 0..32 {
            if bitcount[i] % 3 == 1 {
                ret |= 1 << i;
            }
        }

        ret
    }
}