Problem:
Score : 500 points

Problem Statement

Given are N integers A_1,\ldots,A_N.

We will choose exactly K of these elements. Find the maximum possible product of the chosen elements.

Then, print the maximum product modulo (10^9+7), using an integer between 0 and 10^9+6 (inclusive).

Constraints

1 \leq K \leq N \leq 2\times 10^5

|A_i| \leq 10^9

Input

Input is given from Standard Input in the following format:

N K
A_1 \ldots A_N

Output

Print the maximum product modulo (10^9+7), using an integer between 0 and 10^9+6 (inclusive).

Sample Input 1

4 2
1 2 -3 -4

Sample Output 1

12

The possible products of the two chosen elements are 2, -3, -4, -6, -8, and 12, so the maximum product is 12.

Sample Input 2

4 3
-1 -2 -3 -4

Sample Output 2

1000000001

The possible products of the three chosen elements are -24, -12, -8, and -6, so the maximum product is -6.

We print this value modulo (10^9+7), that is, 1000000001.

Sample Input 3

2 1
-1 1000000000

Sample Output 3

1000000000

The possible products of the one chosen element are -1 and 1000000000, so the maximum product is 1000000000.

Sample Input 4

10 10
1000000000 100000000 10000000 1000000 100000 10000 1000 100 10 1

Sample Output 4

999983200

Be sure to print the product modulo (10^9+7).

Solution in Rust (1.42.0) (judge verdict: Runtime Error)

use std::cmp::*;
use std::io::*;
use std::ops::*;
use utils::*;

pub fn main() {
    let i = stdin();
    let mut o = Vec::new();
    run(i.lock(), &mut o).unwrap();
    stdout().write_all(&o).unwrap();
}

pub fn run(i: impl BufRead, o: &mut impl Write) -> std::io::Result<()> {
    let mut i = CpReader::new(i);
    let (n, k) = i.read::<(usize, usize)>();
    let a = i.read_vec::<i64>(n);
    writeln!(o, "{}", solve(a, k))?;
    Ok(())
}
fn solve(a: Vec<i64>, mut k: usize) -> usize {
    let mut positives: Vec<usize> = a
        .iter()
        .filter(|&&a| a > 0)
        .cloned()
        .map(|a| a as usize)
        .collect();
    let mut negatives: Vec<usize> = a
        .iter()
        .filter(|&&a| a < 0)
        .cloned()
        .map(|a| -a as usize)
        .collect();
    positives.sort();
    negatives.sort();

    let mut ans = Mod::new(1);
    if can_positive(positives.len(), negatives.len(), k) {
        if k % 2 == 1 {
            ans *= positives.pop().unwrap();
            k -= 1;
        }
        let mut vs0 = merge_near(positives);
        let mut vs1 = merge_near(negatives);
        k = k / 2;

        for _ in 0..k {
            match (vs0.pop(), vs1.pop()) {
                (Some(v0), Some(v1)) => {
                    if v0 < v1 {
                        ans *= v1;
                        vs0.push(v0);
                    } else {
                        ans *= v0;
                        vs1.push(v1);
                    }
                }
                (Some(v), None) => ans *= v,
                (None, Some(v)) => ans *= v,
                _ => unreachable!(),
            }
        }
        ans.0
    } else if a.len() - positives.len() - negatives.len() > 0 {
        0
    } else {
        positives.reverse();
        negatives.reverse();

        ans *= utils::MOD_BASE - negatives.pop().unwrap();
        k -= 1;
        if k % 2 == 1 {
            ans *= positives.pop().unwrap();
            k -= 1;
        }
        let mut vs0 = merge_near(positives);
        let mut vs1 = merge_near(negatives);
        k = k / 2;
        for _ in 0..k {
            match (vs0.pop(), vs1.pop()) {
                (Some(v0), Some(v1)) => {
                    if v0 > v1 {
                        ans *= v1;
                        vs0.push(v0);
                    } else {
                        ans *= v0;
                        vs1.push(v1);
                    }
                }
                (Some(v), None) => ans *= v,
                (None, Some(v)) => ans *= v,
                _ => unreachable!(),
            }
        }
        ans.0
    }
}
fn can_positive(positives: usize, negatives: usize, k: usize) -> bool {
    if positives >= k {
        return true;
    }
    let mut n = k - positives;
    if k % 2 == 1 {
        if positives == 0 {
            return false;
        }
        n += 1;
    }
    n >= negatives
}
fn merge_near(mut s: Vec<usize>) -> Vec<usize> {
    let mut r = Vec::new();
    while let (Some(v0), Some(v1)) = (s.pop(), s.pop()) {
        r.push(v0 * v1);
    }
    r
}

pub mod utils {
    use super::*;

    pub struct CpReader<R: BufRead> {
        r: R,
        b: Vec<u8>,
    }
    impl<R: BufRead> CpReader<R> {
        pub fn new(r: R) -> Self {
            CpReader {
                r: r,
                b: Vec::new(),
            }
        }
        pub fn read_word(&mut self) -> &[u8] {
            self.b.clear();
            let mut consume = 0;
            loop {
                self.r.consume(consume);
                let b = self.r.fill_buf().unwrap();
                assert!(b.len() != 0);
                if let Some(p) = b.iter().position(|&x| x == b' ' || x == b'\n') {
                    self.b.extend_from_slice(&b[..p]);
                    consume = p + 1;
                    break;
                }
                self.b.extend_from_slice(b);
                consume = b.len();
            }
            self.r.consume(consume);
            &self.b
        }
        pub fn read_word_str(&mut self) -> &str {
            unsafe { std::str::from_utf8_unchecked(self.read_word()) }
        }

        pub fn read_line(&mut self) -> &[u8] {
            self.b.clear();
            self.r.read_until(b'\n', &mut self.b).unwrap();
            if self.b.last() == Some(&b'\n') {
                &self.b[..self.b.len() - 1]
            } else {
                &self.b
            }
        }
        pub fn read_line_str(&mut self) -> &str {
            unsafe { std::str::from_utf8_unchecked(self.read_line()) }
        }

        pub fn read<T: CpIn>(&mut self) -> T {
            T::read_from(self)
        }

        pub fn read_vec<T: CpIn>(&mut self, n: usize) -> Vec<T> {
            (0..n).map(|_| self.read()).collect()
        }
        pub fn read_iter<'a, T: CpIn>(&'a mut self, n: usize) -> CpIter<'a, R, T> {
            CpIter {
                r: self,
                n: n,
                _pd: Default::default(),
            }
        }
    }
    pub struct CpIter<'a, R: BufRead + 'a, T> {
        r: &'a mut CpReader<R>,
        n: usize,
        _pd: std::marker::PhantomData<fn() -> T>,
    }
    impl<'a, R: BufRead, T: CpIn> Iterator for CpIter<'a, R, T> {
        type Item = T;
        fn next(&mut self) -> Option<T> {
            if self.n == 0 {
                None
            } else {
                self.n -= 1;
                Some(self.r.read())
            }
        }
    }

    pub trait CpIn {
        fn read_from<R: BufRead>(r: &mut CpReader<R>) -> Self;
    }

    impl CpIn for u64 {
        fn read_from<R: BufRead>(r: &mut CpReader<R>) -> Self {
            read_u64_fast(&mut r.r)
        }
    }
    impl CpIn for i64 {
        fn read_from<R: BufRead>(r: &mut CpReader<R>) -> Self {
            read_i64_fast(&mut r.r)
        }
    }
    impl CpIn for char {
        fn read_from<R: BufRead>(r: &mut CpReader<R>) -> Self {
            let b = r.r.fill_buf().unwrap()[0] as char;
            r.r.consume(1);
            let s = r.r.fill_buf().unwrap()[0];
            assert!(s == b' ' || s == b'\n');
            r.r.consume(1);
            b
        }
    }

    macro_rules! cpin_tuple {
        ($($t:ident),*) => {
            impl<$($t: CpIn),*> CpIn for ($($t),*) {
                fn read_from<R: BufRead>(r: &mut CpReader<R>) -> Self {
                    ($($t::read_from(r)),*)
                }
            }
        };
    }

    macro_rules! cpin_cast {
        ($t_self:ty, $t_read:ty) => {
            impl CpIn for $t_self {
                fn read_from<R: BufRead>(r: &mut CpReader<R>) -> Self {
                    <$t_read>::read_from(r) as $t_self
                }
            }
        };
    }
    macro_rules! cpin_parse {
        ($t:ty) => {
            impl CpIn for $t {
                fn read_from<R: BufRead>(r: &mut CpReader<R>) -> Self {
                    r.read_word_str().parse().unwrap()
                }
            }
        };
    }
    cpin_cast!(usize, u64);
    cpin_cast!(u32, u64);
    cpin_cast!(u16, u64);
    cpin_cast!(i32, i64);
    cpin_cast!(i16, i64);
    cpin_cast!(i8, i64);

    cpin_parse!(f64);

    cpin_tuple!(T1, T2);
    cpin_tuple!(T1, T2, T3);
    cpin_tuple!(T1, T2, T3, T4);
    cpin_tuple!(T1, T2, T3, T4, T5);

    fn read_u64_fast<R: BufRead>(r: &mut R) -> u64 {
        let mut value = 0;
        loop {
            let mut idx = 0;
            let buf_len;
            {
                let buf = r.fill_buf().unwrap();
                buf_len = buf.len();
                assert!(buf_len != 0);
                while idx < buf_len && b'0' <= buf[idx] && buf[idx] <= b'9' {
                    value = value * 10 + (buf[idx] - b'0') as u64;
                    idx += 1;
                }
            }
            if idx < buf_len {
                r.consume(idx + 1);
                return value;
            }
            r.consume(idx);
        }
    }
    fn read_i64_fast<R: BufRead>(r: &mut R) -> i64 {
        let (consume, sign) = match r.fill_buf().unwrap()[0] {
            b'+' => (true, 1),
            b'-' => (true, -1),
            _ => (false, 1),
        };
        if consume {
            r.consume(1);
        }
        read_u64_fast(r) as i64 * sign
    }

    #[derive(Clone, Copy)]
    pub struct Mod(pub usize);

    use std::fmt;
    impl fmt::Debug for Mod {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "{:?}", self.0)
        }
    }

    pub const MOD_BASE: usize = 1000000007;
    const MOD_INV_POW: usize = MOD_BASE - 2;

    impl Mod {
        pub fn new(n: usize) -> Self {
            Mod(n % MOD_BASE)
        }
        pub fn pow(self, mut exp: usize) -> Self {
            let mut b = self;
            let mut r = Mod(1);
            while exp != 0 {
                if exp % 2 == 1 {
                    r *= b;
                }
                exp /= 2;
                b = b * b;
            }
            r
        }
        pub fn inverse_element(self) -> Self {
            self.pow(MOD_INV_POW)
        }

        pub fn comb(n: usize, m: usize) -> Mod {
            let mut v0 = Mod::new(1);
            let mut v1 = Mod::new(1);
            for i in 0..min(m, n - m) {
                v0 *= i + 1;
                v1 *= n - i;
            }
            v1 / v0
        }
    }

    macro_rules! mod_op {
        (($ot:ident, $f:ident), ($ot_a:ident, $f_a:ident), $($rhs:ty : $e:expr,)*) => {
            $(
            impl $ot<$rhs> for Mod {
                type Output = Self;
                fn $f(self, rhs: $rhs) -> Self {
                    ($e)(self, rhs)
                }
            }
            impl $ot_a<$rhs> for Mod {
                fn $f_a(&mut self, rhs: $rhs) {
                    *self = ($e)(*self, rhs)
                }
            }
            )*
        }
    }

    mod_op!(
        (Add, add),
        (AddAssign, add_assign),
        Mod : |l: Mod, r: Mod| Mod::new(l.0 + r.0),
        usize : |l: Mod, r: usize| l + Mod::new(r),
    );

    mod_op!(
        (Mul, mul),
        (MulAssign, mul_assign),
        Mod : |l: Mod, r: Mod| Mod::new(l.0 * r.0),
        usize : |l: Mod, r: usize| l * Mod::new(r),
    );

    mod_op!(
        (Sub, sub),
        (SubAssign, sub_assign),
        Mod : |l: Mod, r: Mod| Mod::new(MOD_BASE + l.0 - r.0),
        usize : |l: Mod, r: usize| l - Mod::new(r),
    );

    mod_op!(
        (Div, div),
        (DivAssign, div_assign),
        Mod : |l: Mod, r: Mod| l * r.inverse_element(),
        usize : |l: Mod, r: usize| l / Mod::new(r),
    );

    pub struct PascalsTriangle<T> {
        buf: Vec<T>,
        n: usize,
    }

    impl<T: One> PascalsTriangle<T> {
        pub fn new(n: usize) -> Self {
            let mut s = PascalsTriangle {
                buf: vec![T::one()],
                n: 1,
            };
            while s.n <= n {
                let i = s.buf.len() - s.n;
                s.buf.push(T::one());
                for m in 1..s.n {
                    let v = s.buf[i + m - 1] + s.buf[i + m];
                    s.buf.push(v);
                }
                s.buf.push(T::one());
                s.n += 1;
            }
            s
        }
        pub fn get(&mut self, n: usize, m: usize) -> T {
            debug_assert!(n <= self.n);
            debug_assert!(m <= n);
            self.buf[n * (n + 1) / 2 + m]
        }
    }

    pub struct PascalsTriangleIter<T> {
        buf: Vec<T>,
    }
    impl<T: One> PascalsTriangleIter<T> {
        pub fn new(n: usize) -> Self {
            let mut s = PascalsTriangleIter {
                buf: Vec::with_capacity(n + 1),
            };
            for _ in 0..n + 1 {
                s.next();
            }
            s
        }
        pub fn next(&mut self) {
            for i in (1..self.buf.len()).rev() {
                self.buf[i] = self.buf[i] + self.buf[i - 1];
            }
            self.buf.push(T::one());
        }
        pub fn get(&self) -> &[T] {
            &self.buf
        }
    }

    pub struct ModFacts {
        f: Vec<Mod>,
        rf: Vec<Mod>,
    }
    impl ModFacts {
        pub fn new(n: usize) -> Self {
            let mut s = ModFacts {
                f: vec![Mod(0); n + 1],
                rf: vec![Mod(0); n + 1],
            };

            s.f[0] = Mod(1);
            for i in 1..n + 1 {
                s.f[i] = s.f[i - 1] * i;
            }
            s.rf[n] = s.f[n].inverse_element();
            for i in (1..n + 1).rev() {
                s.rf[i - 1] = s.rf[i] * i;
            }
            s
        }
        pub fn fact(&self, n: usize) -> Mod {
            self.f[n]
        }
        pub fn comb(&self, n: usize, c: usize) -> Mod {
            self.f[n] * self.rf[c] * self.rf[n - c]
        }
    }

    pub trait Zero: Add<Output = Self> + Sized + Copy {
        fn zero() -> Self;
    }
    pub trait One: Zero {
        fn one() -> Self;
    }

    macro_rules! impl_zero {
        ($t:ty, $e:expr) => {
            impl Zero for $t {
                fn zero() -> Self {
                    $e
                }
            }
        }
    }
    macro_rules! impl_one {
        ($t:ty, $e1:expr, $e0:expr) => {
            impl One for $t {
                fn one() -> Self {
                    $e1
                }
            }
            impl_zero!($t, $e0);
        };
    }

    impl_one!(usize, 1, 0);
    impl_one!(u32, 1, 0);
    impl_one!(i32, 1, 0);
    impl_one!(f64, 1.0, 0.0);
    impl_one!(Mod, Mod(1), Mod(0));
}