problem 1 2 4 in common lisp

1 files changed, 23 insertions, 0 deletions

diff --git a/lisp/004-largest_palindrome_product.lisp b/lisp/004-largest_palindrome_product.lisp
new file mode 100644
index 0000000..8cbc7c9
--- /dev/null
+++ b/lisp/004-largest_palindrome_product.lisp
@@ -0,0 +1,23 @@
+;;;; ;;;;;;;;
+;;;; Largest palindrome product
+;;;; Problem 4
+;;;;
+;;;; A palindromic number reads the same both ways.
+;;;; The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
+;;;; Find the largest palindrome made from the product of two 3-digit numbers.
+;;;; ;;;;;;;;
+
+(defun palindrome-p (n)
+  (equal (write-to-string n) (reverse (write-to-string n))))
+
+(print (palindrome-p 32))
+
+(defvar *largest* 0)
+
+(loop :for x :from 999 :downto 1 :do
+      (loop :for y :from 999 :downto 1 :do
+            (when (and (palindrome-p (* x y)) (> (* x y) *largest*))
+              (print (* x y))
+              (setf *largest* (* x y)))))
+
+(print *largest*)