adding old leetcode solutions

1 files changed, 28 insertions, 0 deletions

diff --git a/leetcode/lc-43-multiply-strings.rkt b/leetcode/lc-43-multiply-strings.rkt
new file mode 100644
index 0000000..dac8c31
--- /dev/null
+++ b/leetcode/lc-43-multiply-strings.rkt
@@ -0,0 +1,28 @@
+#lang racket
+
+(define/contract (char-digit->integer c)
+  (-> char? integer?)
+  (- (char->integer c) 48))
+
+(define/contract (integer->string-digit n)
+  (-> integer? string?)
+  (string (integer->char (+ n 48))))
+
+(define/contract (number->string1 n [acc ""])
+  (->* (integer?) (string?) string?)
+  (cond [(and (= n 0) (equal? acc "")) "0"]
+        [(= n 0) acc]
+        [else (number->string1
+               (quotient n 10)
+               (string-append (integer->string-digit (remainder n 10)) acc))]))
+
+(define/contract (multiply num1 num2)
+  (-> string? string? string?)
+  (define multiplication-steps
+    (for/list ([n1 (in-string num1)]
+               [place1 (in-range (sub1 (string-length num1)) -1 -1)])
+      (for/list ([n2 (in-string num2)]
+                 [place2 (in-range (sub1 (string-length num2)) -1 -1)])
+        (apply * (append (map char-digit->integer (list n1 n2))
+                         (list (expt 10 place1) (expt 10 place2)))))))
+  (number->string1 (apply + (flatten multiplication-steps))))
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