use `fold` for single_number Solution1

[ChrisLinn]

https://github.com/rust-interview/rust-leetcode-solutions/blob/master/kamyu104/src/single_number_ii.rs#L12

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=35f73d98e6255726e489147a0e8206d5

// Time:  O(n)
// Space: O(1)

pub struct Solution {}

impl Solution {
    pub fn single_number(nums: Vec<i32>) -> i32 {
        let (one, _) = nums.iter().fold((0, 0), |(one, two), num| {
            (
                (!num & one) | (num & !one & !two),
                (!num & two) | (num & one),
            )
        });
        one
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_add() {
        assert_eq!(Solution::single_number(vec![2, 2, 3, 2]), 3);
        assert_eq!(Solution::single_number(vec![0, 1, 0, 1, 0, 1, 99]), 99);
        assert_eq!(Solution::single_number(vec![0, 0, 0, 1, 1, 1, 5]), 5);
    }
}