Add purescript solution for chapt 1

01_introduction_to_algorithms/Purescript/BinarySearch.purs

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+module GrokkingAlgos.BinarySearch where
+
+import Prelude
+import Data.Array ((!!))
+import Data.Array as Array
+import Data.Maybe (Maybe(..))
+import Effect (Effect)
+import Effect.Class.Console (logShow)
+
+-- | Binary Search - input is a sorted list of elements
+-- | Big o notation - log n
+-- | Traveling salesman - O (n!)
+binarysearch :: Int -> Array Int -> Int -> Int -> Maybe Int
+binarysearch x arr low high
+  | low > high = Nothing
+  | otherwise =
+    let
+      mid = (high + low) / 2
+    in
+      arr !! mid
+        >>= case _ of
+            item
+              | item == x -> Just mid
+            item
+              | item > x -> binarysearch x arr low (mid - 1)
+            item
+              | item < x -> binarysearch x arr (mid + 1) high
+            _ -> Nothing
+
+find :: Int -> Array Int -> Maybe Int
+find x arr = binarysearch x arr low high
+  where
+  low = 0
+
+  high = (Array.length arr) - 1
+
+main :: Effect Unit
+main = do
+  logShow $ find 20 [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20 ]