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|
fun member x [] = false
| member x (y::ys) = if x=y then true else (member x ys);
(* check for (0,q) and (k, qpos[col=k]) where k=1...
http://www.nct9.ne.jp/m_hiroi/func/haskell06.html
0 1 2 -->
*-------------
1| . . . . . .
2| . . . -3. . 5 - 3 = 2
3| . . -2. . . 5 - 2 = 3
4| . -1. . . . 5 - 1 = 4
5| Q . . . . . Q position is 5
6| . +1. . . . 5 + 1 = 6
7| . . +2. . . 5 + 2 = 7
8| . . . +3. . 5 + 2 = 8
*-------------
*)
fun threat k m [] = false
| threat k m (x::xs) =
if (k = x-m) orelse (k = m-x) then true
else (threat (k+1) m xs);
fun queen1 0 b n = []
| queen1 m b n =
if (member m b) orelse (threat 1 m b) then (queen1 (m-1) b n)
else if (length b) = (n-1) then [m::b]@(queen1 (m-1) b n)
else (queen1 n (m::b) n) @ (queen1 (m-1) b n);
fun queen n = queen1 n [] n;
val n = 12;
length (queen n);
|
