use std::collections::HashMap;

// I'm fairly sure there's a better more mathsy way of doing this but I can't work it out yet.
pub fn single_number(nums: Vec<i32>) -> i32 {
    let mut hm = HashMap::<i32, i32>::new();
    for n in nums {
        if hm.contains_key(&n) {
            hm.remove(&n);
        } else {
            hm.insert(n, 0);
        }
    } 
    match hm.keys().next() {
        Some(i) => *i,
        _ => 0
    }
}

pub fn main() {
    println!("Expected: 1");
    println!("Got: {}", single_number(vec![1, 2, 3, 4, 3, 4, 2]));
}