Merge pull request lambda-study-group#4 from jvrn3/main

day 1

Author: souenzzo

2020/jvrn3/racket/day1.rkt

@@ -0,0 +1,39 @@
+#lang racket
+
+; receives a function and a list of integers
+; the function f determines if it's gonna be a sum of two or three numbers
+(define (find-2020 f xs)
+  (let ([value (f (first xs) (rest xs))])
+    (if (number? value) value (find-2020 f (rest xs)))))
+   
+; returns the multiplication of two numbers which the sum is equal to 2020
+; if there is no such a pair, returns false
+(define (check-two x xs)
+  (cond
+    [(null? xs) #f]
+    [(equal? (+ x (first xs)) 2020)
+     (* x (first xs))]
+    [else
+     (check-two x (rest xs))]))
+
+
+; same as check-two, but for three numbers
+(define (check-three x xs)
+  (cond
+    [(null? xs) #f]
+    [else 
+     (let ([value (compare-three x (first xs) xs)])
+       (if (number? value) value (check-three x (rest xs))))]))
+
+
+; if there's some number from xs that x + y + number is equal to 2020, returns their multiplication
+; otherwise returns false
+(define (compare-three x y xs)
+  (cond
+    [(null? xs) #f]
+    [(equal? (+ x y (first xs)) 2020)
+     (* x y (first xs))]
+    [else (compare-three x y (rest xs))]))
+
+
+