study(0137): use bitwise ops

[massivebird]

I'm having trouble breaking down the double-mask bitwise ops approach. Here's one ripped from Leetcode's best runtime solutions:

impl Solution {
    pub fn single_number(nums: Vec<i32>) -> i32 {
        let mut ones = 0;
        let mut twos = 0;
        
        for n in nums {
            ones = (ones ^ n) & !twos;
            twos = (twos ^ n) & !ones;
        }
        
        ones
    }
}

Something about how one mask prevent certain bits from being set in the other, but their intermittent values are throwing me off. I don't really understand what they're supposed to concretely represent, or how this approach was derived.