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//! 累積和
pub fn cumsum(v: &[i64]) -> Vec<i64> {
    (0..1)
        .chain(v.iter().scan(0, |c, &x| {
            *c += x;
            Some(*c)
        }))
        .collect()
}

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_cumsum() {
        let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
        let cumsum = cumsum(&v);
        assert_eq!(&cumsum, &[0, 1, 3, 6, 10, 15, 21, 28, 36, 45, 55]);
    }
}