LISP回溯

Question

有人可以指导我或解释如何在LISP中进行回溯吗？任何示例或链接将不胜感激.我确实尝试过谷歌,但是没有一个能够让我理解的简单例子.

谢谢

Answer 1

典型的方法是将非可变状态传递给调用堆栈,辅助函数将当前状态返回到新的状态为"伪"突变.

一个可能的(虽然相当次优)数独求解器将是:

;;; Use a list of 81 integers to represent a sudoku board,
;;; each number 1-9 represents itself, 0 represents a blank
(defun sudoku-solver (board)
  (cond ((notany #'zerop board)
     (if (sudoku-solved-p board)
         board
         nil))
    (t (let ((positions (sudoku-all-blanks board)))
         (loop for position in positions
          do (loop for number in '(1 2 3 4 5 6 7 8 9)
              do (let ((result (sudoku-solver
                        (sudoku-set board
                            position
                            number))))
                   (when result
                 (return-from sudoku-solver result)))))))))

这将自动回溯,直到找到解决方案.我已经跳过了使用支持代码来模糊演示,该代码将它从演示变为实际工作代码.