4.44

exercises/04/4.44.scm

@@ -0,0 +1,32 @@
+; 本题和 2.42 表示方法一样，用一个list表示一种解法，比如四皇后问题，解法(A B C D)表示，第一个皇后在位置A，第二个皇后在位置B，第三个皇后在位置C，第四个皇后在位置D，用list的位置表示了皇后的次序。
+
+; https://github.com/jiacai2050/sicp/blob/master/exercises/02/2.42_2.43.md#242
+
+
+(define (check-current-diagonal? current-postion rest distance-to-current)
+  (if (null? rest)
+    #f
+    (or (= (- current-postion distance-to-current) (car rest))
+        (= (+ current-postion distance-to-current) (car rest))
+        (check-current-diagonal? current-postion (cdr rest) (+ distance-to-current 1)))))
+
+(define (in-diagonal? positions)
+  (if (null? positions)
+    #f
+    (or (in-diagonal (car positions) (cdr positions) 1)
+        (in-diagonal? (cdr positions)))))
+
+(define (collide? positions)
+  (or (not (distinct? positions))
+      (in-diagonal? positions)))
+
+(define (eight-queen)
+  (let ((q1 (amb 1 2 3 4 5 6 7 8))
+        (q2 (amb 1 2 3 4 5 6 7 8))
+        (q3 (amb 1 2 3 4 5 6 7 8))
+        (q4 (amb 1 2 3 4 5 6 7 8))
+        (q5 (amb 1 2 3 4 5 6 7 8))
+        (q6 (amb 1 2 3 4 5 6 7 8))
+        (q7 (amb 1 2 3 4 5 6 7 8))
+        (q8 (amb 1 2 3 4 5 6 7 8)))
+    (require (not (collide? (list q1 q2 q3 q4 q5 q6 q7 q8))))))