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September 2017 Update

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Ingy döt Net 2017-09-23 10:01:46 +02:00
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'N-queens problem - non recursive & structured - vb.net - 26/02/2017
Module Mod_n_queens
Sub n_queens()
Const l = 15 'number of queens
Const b = False 'print option
Dim a(l), s(l), u(4 * l - 2)
Dim n, m, i, j, p, q, r, k, t, z
Dim w As String
For i = 1 To UBound(a) : a(i) = i : Next i
For n = 1 To l
m = 0
i = 1
j = 0
r = 2 * n - 1
Do
i = i - 1
j = j + 1
p = 0
q = -r
Do
i = i + 1
u(p) = 1
u(q + r) = 1
z = a(j) : a(j) = a(i) : a(i) = z 'Swap a(i), a(j)
p = i - a(i) + n
q = i + a(i) - 1
s(i) = j
j = i + 1
Loop Until j > n Or u(p) Or u(q + r)
If u(p) = 0 Then
If u(q + r) = 0 Then
m = m + 1 'm: number of solutions
If b Then
Debug.Print("n=" & n & " m=" & m) : w = ""
For k = 1 To n
For t = 1 To n
w = w & If(a(n - k + 1) = t, "Q", ".")
Next t
Debug.Print(w)
Next k
End If
End If
End If
j = s(i)
Do While j >= n And i <> 0
Do
z = a(j) : a(j) = a(i) : a(i) = z 'Swap a(i), a(j)
j = j - 1
Loop Until j < i
i = i - 1
p = i - a(i) + n
q = i + a(i) - 1
j = s(i)
u(p) = 0
u(q + r) = 0
Loop
Loop Until i = 0
Debug.Print(n & vbTab & m) 'number of queens, number of solutions
Next n
End Sub 'n_queens
End Module