Garbage-Collection

N = 25 -- board size

-- check whether position (n, c) is free from attacks
function isplaceok (a, n, c)
    for i = 1, n - 1 do
        if (a[i] == c) or                    -- cumulative column
                (a[i] - i == c - n) or       -- ↘ diagonal (cumulative)
                (a[i] + i == c + n) then     -- ↙ diagonal (cumulative)
            return false
        end
    end
    return true
end

-- print a board
function printsolution (a)
    for i = 1, N do         -- for each row
        for j = 1, N do     -- and for each column
            -- write "X" or "-" plus a space
            io.write(a[i] == j and "X" or "-", " ")
        end
        io.write("\n")
    end
    io.write("\n")
end

-- add to board 'a' all queens from 'n' to 'N'
function addqueen (a, n)
    if n > N then           -- all queens have been placed?
        printsolution(a)
        return true
    else    -- try to place n-th quen
        for c = 1, N do
            if isplaceok(a, n, c) then
                a[n] = c    -- place n-th queen at column 'c'
                if addqueen(a, n + 1) then
                    return true
                end
            end
        end
    end
end

addqueen({},1)

--[[
this code is really quite confusing.

firstly it uses backtracking, secondly the table data structure is non-trivial.
the notation a[n] == c, checks if there is a queen at co-ordinate (n,c).

it works by not using subarrays, and just indexing the column of each queen!

the recursion is fine, the base case exists at the start of the addqueen function, and we
loop through all the rows, adding queens where you can, and then backtracking through the
tree when we've ended up in a non-solvable situation.

the confusion for me arose in the isplaceok function, where I thought we would loop over
the entire NxN board; how naive! instead if we are considering the nth queen, we have only
placed 1..n-1 queens, and only need to check those squares!

finally, the crux of the magic is in 

        if (a[i] == c) or                    -- cumulative column
                (a[i] - 1 == c - n) or       -- ↘ diagonal (cumulative)
                (a[i] + 1 == c + n) then     -- ↙ diagonal (cumulative)

where it took me some time to understand that the checks were cumulative and that indeed
the indexing wizardry checked the cumulative column / diagonal / anti-diagonal.

note that there are 92 solutions to the 8x8 problem.

furthermore, the alternatives are the brute-force approach, where 8^8 boards are spawned and we need to check all column / diagonal and anti-diagonal violations.

there is also the permutation approach where you can just eliminate the row and col the queen was placed on and then continue your permutations for the n-1 sized square board. this avoids some cases by construction.

the true brute-force approach is O(N^N), whilst permutation is O(N!) and lastly backtracking is O(N!) worst case (but this is much better in practice).
--]]