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hyphenrf's solution

to Binary Search Tree in the OCaml Track

Published at Jul 08 2020 · 0 comments
Instructions
Test suite
Solution

Insert and search for numbers in a binary tree.

When we need to represent sorted data, an array does not make a good data structure.

Say we have the array [1, 3, 4, 5], and we add 2 to it so it becomes [1, 3, 4, 5, 2] now we must sort the entire array again! We can improve on this by realizing that we only need to make space for the new item [1, nil, 3, 4, 5], and then adding the item in the space we added. But this still requires us to shift many elements down by one.

Binary Search Trees, however, can operate on sorted data much more efficiently.

A binary search tree consists of a series of connected nodes. Each node contains a piece of data (e.g. the number 3), a variable named left, and a variable named right. The left and right variables point at nil, or other nodes. Since these other nodes in turn have other nodes beneath them, we say that the left and right variables are pointing at subtrees. All data in the left subtree is less than or equal to the current node's data, and all data in the right subtree is greater than the current node's data.

For example, if we had a node containing the data 4, and we added the data 2, our tree would look like this:

  4
 /
2

If we then added 6, it would look like this:

  4
 / \
2   6

If we then added 3, it would look like this

   4
 /   \
2     6
 \
  3

And if we then added 1, 5, and 7, it would look like this

      4
    /   \
   /     \
  2       6
 / \     / \
1   3   5   7

Source

Josh Cheek https://twitter.com/josh_cheek

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

test.ml

(* binary-search-tree - 1.0.0 *)
open Base
open OUnit2
open Binary_search_tree

let result_to_string f = function
  | Error m -> Printf.sprintf "Error \"%s\"" m
  | Ok x -> f x |> Printf.sprintf "Some %s"

let ae exp got _test_ctxt =
  assert_equal ~printer:(result_to_string Int.to_string) exp got

let intlist_to_string l = 
  List.map l ~f:Int.to_string
  |> List.intersperse ~sep:"; "
  |> List.fold ~init:"" ~f:(^) 
  |> fun s -> "[" ^ s ^ "]"

let ael exp got _test_ctxt =
  assert_equal ~printer:intlist_to_string exp got

let tests = 
  let t4 = empty |> insert 4 in 
  let t42 = t4 |> insert 2 in  
  let l2 =  t42 |> left in  
  let t44 = t4 |> insert 4 in 
  let l4 =  t44 |> left in 
  let t45 = t4 |> insert 5 in
  let r5 = t45 |> right in  
  let t4261357 = t42 |> insert 6 |> insert 1 |> insert 3 |> insert 5 |> insert 7 in
  let t2 = empty |> insert 2 in
  let t21 = t2 |> insert 1 in
  let t22 = t2 |> insert 2 in
  let t23 = t2 |> insert 3 in
  let t213675 = t21 |> insert 3 |> insert 6 |> insert 7 |> insert 5 in
  [
    "data is retained" >:: ae (Ok 4) (value t4);
    "smaller number at left node 1" >:: ae (Ok 4) (value t42);
    "smaller number at left node 2" >:: ae (Ok 2) (Result.bind l2 ~f:value);
    "same number at left node 1" >:: ae (Ok 4) (value t44);
    "same number at left node 2" >:: ae (Ok 4) (Result.bind l4 ~f:value);
    "greater number at right node 1" >:: ae (Ok 4) (value t45);
    "greater number at right node 2" >:: ae (Ok 5) (Result.bind r5 ~f:value);
    "can create complex tree 1" >:: ae (Ok 4) (value t4261357);
    "can create complex tree 2" >:: ae (Ok 2) (Result.bind (t4261357 |> left) ~f:value);
    "can create complex tree 3" >:: ae (Ok 1) (Result.bind (Result.bind (t4261357 |> left) ~f:left) ~f:value);
    "can create complex tree 4" >:: ae (Ok 3) (Result.bind (Result.bind (t4261357 |> left) ~f:right) ~f:value);
    "can create complex tree 5" >:: ae (Ok 6) (Result.bind (t4261357 |> right) ~f:value);
    "can create complex tree 6" >:: ae (Ok 5) (Result.bind (Result.bind (t4261357 |> right) ~f:left) ~f:value);
    "can create complex tree 7" >:: ae (Ok 7) (Result.bind (Result.bind (t4261357 |> right) ~f:right) ~f:value);
    "can sort single number" >:: ael [2] (to_list t2);
    "can sort if second number is smaller than first" >:: ael [1;2] (to_list t21);
    "can sort if second number is same as first" >:: ael [2;2] (to_list t22);
    "can sort if second number is greater than first" >:: ael [2;3] (to_list t23);
    "can sort complex tree" >:: ael [1; 2; 3; 5; 6; 7] (to_list t213675);
  ]

let () =
  run_test_tt_main ("binary-search-tree tests" >::: tests)
(* open Base *)

type bst =
   | Leaf
   | Node of int * bst * bst


let (++) = List.append


let empty = Leaf

let value tree = match tree with
    | Leaf           -> Error "Empty tree."
    | Node (k, _, _) -> Ok k

let left tree = match tree with
    | Leaf           -> Error "Empty tree."
    | Node (_, l, _) -> Ok l

let right tree = match tree with
    | Leaf           -> Error "Empty tree."
    | Node (_, _, r) -> Ok r

let rec insert i tree = match tree with
    | Leaf           -> Node (i, Leaf, Leaf)
    | Node (k, l, r) -> if i <= k 
            then Node (k, insert i l, r)
            else Node (k, l, insert i r)

let rec to_list tree = match tree with
    | Leaf           -> []
    | Node (k, l, r) -> (to_list l) ++ (k::to_list r)

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