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 Fortran (GNU Fortran 9.2.1) (judge verdict: Wrong Answer)

module merge_sort_mod
    use,intrinsic :: iso_fortran_env
    implicit none
    private
    public:: merge_sort, double_merge_sort
    interface merge_sort
        module procedure ms32, ms64
    end interface

    interface double_merge_sort
        module procedure msd3232, msd6464
    end interface

contains
    recursive subroutine ms32(ar, fst, lst)
        integer(int32),intent(inout):: ar(:)
        integer(int32),intent(in):: fst,lst
        integer(int32):: mdl

        if (lst-fst < 2) then
            if (ar(fst) > ar(lst)) call swap32(ar(fst),ar(lst))
            return
        end if

        mdl = (fst+lst)/2
        call ms32(ar, fst, mdl)
        call ms32(ar, mdl+1, lst)
        call merge32(ar, fst, mdl, lst)
    end subroutine


    subroutine merge32(ar, fst, mdl, lst)
        integer(int32),intent(inout):: ar(:)
        integer(int32),intent(in):: fst, mdl, lst
        integer(int32),allocatable:: tmp(:)
        integer(int32):: li, ri, ti

        allocate(tmp(lst-fst+1))

        li=fst
        ri=mdl+1   
        ti=1

        do while (li <= mdl .and. ri <= lst)
            if (ar(li) <= ar(ri)) then
                tmp(ti) = ar(li)
                li=li+1
            else
                tmp(ti) = ar(ri)
                ri=ri+1
            end if
            ti=ti+1
        end do

        if (li <= mdl) then
            tmp(ti:) = ar(li:mdl)
        else
            tmp(ti:) = ar(ri:lst)
        end if

        ar(fst:lst) = tmp(:)
        deallocate(tmp)
    end subroutine


    subroutine swap32(x,y)
        integer(int32),intent(inout):: x,y
        integer(int32):: tmp
        tmp = x
        x = y
        y = tmp
    end subroutine


    recursive subroutine ms64(ar, fst, lst)
        integer(int64),intent(inout):: ar(:)
        integer(int64),intent(in):: fst,lst
        integer(int64):: mdl

        if (lst-fst < 2) then
            if (ar(fst) > ar(lst)) call swap64(ar(fst),ar(lst))
            return
        end if

        mdl = (fst+lst)/2
        call ms64(ar, fst, mdl)
        call ms64(ar, mdl+1, lst)
        call merge64(ar, fst, mdl, lst)
    end subroutine


    subroutine merge64(ar, fst, mdl, lst)
        integer(int64),intent(inout):: ar(:)
        integer(int64),intent(in):: fst, mdl, lst
        integer(int64),allocatable:: tmp(:)
        integer(int64):: li, ri, ti

        allocate(tmp(lst-fst+1))

        li=fst
        ri=mdl+1   
        ti=1

        do while (li <= mdl .and. ri <= lst)
            if (ar(li) <= ar(ri)) then
                tmp(ti) = ar(li)
                li=li+1
            else
                tmp(ti) = ar(ri)
                ri=ri+1
            end if
            ti=ti+1
        end do

        if (li <= mdl) then
            tmp(ti:) = ar(li:mdl)
        else
            tmp(ti:) = ar(ri:lst)
        end if

        ar(fst:lst) = tmp(:)
        deallocate(tmp)
    end subroutine


    subroutine swap64(x,y)
        integer(int64),intent(inout):: x,y
        integer(int64):: tmp
        tmp = x
        x = y
        y = tmp
    end subroutine

    recursive subroutine msd3232(ar1, ar2, fst, lst)
        integer(int32),intent(inout):: ar1(:),ar2(:)
        integer(int32),intent(in):: fst,lst
        integer(int32):: mdl

        if (lst - fst < 2) then
            if (ar1(fst) > ar1(lst)) then
                call swap32(ar1(fst), ar1(lst))
                call swap32(ar2(fst), ar2(lst))
            end if
            return
        end if

        mdl = (fst+lst)/2

        call msd3232(ar1,ar2,fst,mdl)
        call msd3232(ar1,ar2,mdl+1,lst)
        call merged3232(ar1,ar2,fst,mdl,lst)
    end subroutine


    subroutine merged3232(ar1,ar2,fst,mdl,lst)
        integer(int32),intent(inout):: ar1(:),ar2(:)
        integer(int32),intent(in):: fst,mdl,lst
        integer(int32),allocatable:: t1(:),t2(:)
        integer(int32):: li,ri,ti

        allocate(t1(lst-fst+1), t2(lst-fst+1))

        li=fst
        ri=mdl+1
        ti=1

        do while(li <= mdl .and. ri <= lst)
            if (ar1(li) <= ar1(ri)) then
                t1(ti) = ar1(li) 
                t2(ti) = ar2(li)
                li=li+1
            else
                t1(ti) = ar1(ri)
                t2(ti) = ar2(ri)
                ri=ri+1
            end if
            ti=ti+1
        end do

        if (li <= mdl) then
            t1(ti:) = ar1(li:mdl)
            t2(ti:) = ar2(li:mdl)
        else
            t1(ti:) = ar1(ri:lst)
            t2(ti:) = ar2(ri:lst)
        end if

        ar1(fst:lst) = t1(:)
        ar2(fst:lst) = t2(:)

        deallocate(t1,t2)
    end subroutine


    recursive subroutine msd6464(ar1, ar2, fst, lst)
        integer(int64),intent(inout):: ar1(:),ar2(:)
        integer(int64),intent(in):: fst,lst
        integer(int64):: mdl

        if (lst - fst < 2) then
            if (ar1(fst) > ar1(lst)) then
                call swap64(ar1(fst), ar1(lst))
                call swap64(ar2(fst), ar2(lst))
            end if
            return
        end if

        mdl = (fst+lst)/2

        call msd6464(ar1,ar2,fst,mdl)
        call msd6464(ar1,ar2,mdl+1,lst)
        call merged6464(ar1,ar2,fst,mdl,lst)
    end subroutine


    subroutine merged6464(ar1,ar2,fst,mdl,lst)
        integer(int64),intent(inout):: ar1(:),ar2(:)
        integer(int64),intent(in):: fst,mdl,lst
        integer(int64),allocatable:: t1(:),t2(:)
        integer(int64):: li,ri,ti

        allocate(t1(lst-fst+1), t2(lst-fst+1))

        li=fst
        ri=mdl+1
        ti=1

        do while(li <= mdl .and. ri <= lst)
            if (ar1(li) <= ar1(ri)) then
                t1(ti) = ar1(li) 
                t2(ti) = ar2(li)
                li=li+1
            else
                t1(ti) = ar1(ri)
                t2(ti) = ar2(ri)
                ri=ri+1
            end if
            ti=ti+1
        end do

        if (li <= mdl) then
            t1(ti:) = ar1(li:mdl)
            t2(ti:) = ar2(li:mdl)
        else
            t1(ti:) = ar1(ri:lst)
            t2(ti:) = ar2(ri:lst)
        end if

        ar1(fst:lst) = t1(:)
        ar2(fst:lst) = t2(:)

        deallocate(t1,t2)
    end subroutine
end module

program main
    use,intrinsic :: iso_fortran_env
    use merge_sort_mod
    implicit none
    integer(int64):: n,k,i,ans=1,l,n2
    integer(int64),parameter:: md=1000000007
    integer(int64), allocatable:: a(:),ma(:),mb(:)

    read*, n,k
    allocate(a(0:n),source=1_8)
    allocate(ma((k+1)/2),source=1_8)
    n2 = (n-k+1)/2
    allocate(mb(n2),source=1_8)
    read*, a(1:n)

    call merge_sort(a(1:n),1_8,n)
    a(1:n) = a(n:1:-1)
    l=1
    do i=k-1,0,-2
        ma(l) = a(i)*a(i+1)
        l=l+1
    end do

    call merge_sort(ma(:),1_8,(k+1)/2)

    if (n2==0) then
        do i=1,(k+1)/2
            ma(i) = mod(ma(i),md)
            ans=mod(ans*ma(i),md)
        end do
        print'(i0)', modulo(ans,md)
        stop
    end if


    l=1
    a(k) = 1
    do i=n-1,k,-2
        mb(l)=a(i)*a(i+1)
        l=l+1
    end do
    call merge_sort(mb(:), 1_8, n2)
    mb(:) = mb(n2:1:-1)


    l=1
    do i=1,(k+1)/2
        if (ma(i) < mb(l)) then
            ma(i) = mb(l)
            l=l+1
        end if
        ma(i) = modulo(ma(i),md)
        if (l>n2) exit
    end do
    do i=1,(k+1)/2
        ans=modulo(ans*ma(i),md)
    end do
    print'(i0)', ans
end program main