#[derive(Debug)]
struct NumArray {
    prefix_sums: Vec<i32>,
}

impl NumArray {
    fn new(nums: Vec<i32>) -> Self {
        let mut total = nums.iter().sum::<i32>();

        let mut prefix_sums: Vec<i32> = nums
            .iter()
            .scan(total, |total, &x| {
                let prev = *total;
                *total -= x;

                Some(prev)
            })
            .collect();
        prefix_sums.push(0);

        Self { prefix_sums }
    }

    fn sum_range(&self, left: i32, right: i32) -> i32 {
        self.prefix_sums[left as usize] - self.prefix_sums[right as usize + 1]
    }
}

fn main() {}

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

    #[test]
    fn example_1() {
        let num_arr = NumArray::new(vec![-2, 0, 3, -5, 2, -1]);

        dbg!("{:?}", &num_arr);

        assert_eq!(num_arr.sum_range(0, 2), 1);
        assert_eq!(num_arr.sum_range(2, 5), -1);
        assert_eq!(num_arr.sum_range(0, 5), -3);
    }
}