[김한기] Week 2 solutions

invert-binary-tree/han.exs

@@ -0,0 +1,37 @@
+# Definition for a binary tree node.
+#
+# defmodule TreeNode do
+#   @type t :: %__MODULE__{
+#           val: integer,
+#           left: TreeNode.t() | nil,
+#           right: TreeNode.t() | nil
+#         }
+#   defstruct val: 0, left: nil, right: nil
+# end
+
+defmodule Solution do
+  @spec invert_tree(root :: TreeNode.t | nil) :: TreeNode.t | nil
+  def invert_tree(nil), do: nil
+
+  def invert_tree(root) do
+    invert(root, root.left, root.right)
+  end
+
+  defp invert(parent, nil, nil), do: parent
+
+  defp invert(parent, left, nil) do
+    %TreeNode{parent | left: nil, right: invert(left, left.left, left.right)}
+  end
+
+  defp invert(parent, nil, right) do
+    %TreeNode{parent | left: invert(right, right.left, right.right), right: nil}
+  end
+
+  defp invert(parent, left, right) do
+    tmp = left
+
+    %TreeNode{parent |
+      left: invert(right, right.left, right.right),
+      right: invert(tmp, tmp.left, tmp.right)}
+  end
+end

linked-list-cycle/han.rb

@@ -0,0 +1,26 @@
+# Definition for singly-linked list.
+# class ListNode
+#     attr_accessor :val, :next
+#     def initialize(val)
+#         @val = val
+#         @next = nil
+#     end
+# end
+
+# @param {ListNode} head
+# @return {Boolean}
+def hasCycle(head)
+    return false unless head && head.next
+
+    slow = head
+    fast = head.next
+
+    loop do
+      break true if slow == fast
+
+      slow = slow.next
+      fast = fast&.next&.next
+
+      break false unless fast
+    end
+  end

merge-two-sorted-lists/han.exs

@@ -0,0 +1,24 @@
+# Definition for singly-linked list.
+#
+# defmodule ListNode do
+#   @type t :: %__MODULE__{
+#           val: integer,
+#           next: ListNode.t() | nil
+#         }
+#   defstruct val: 0, next: nil
+# end
+
+defmodule Solution do
+    @spec merge_two_lists(list1 :: ListNode.t | nil, list2 :: ListNode.t | nil) :: ListNode.t | nil
+  def merge_two_lists(nil, nil), do: nil
+  def merge_two_lists(nil, list2), do: list2
+  def merge_two_lists(list1, nil), do: list1
+
+  def merge_two_lists(list1, list2) when list1.val >= list2.val do
+    %ListNode{list2 | next: merge_two_lists(list1, list2.next)}
+  end
+
+  def merge_two_lists(list1, list2) do
+    %ListNode{list1 | next: merge_two_lists(list1.next, list2)}
+  end
+end

reverse-linked-list/han.exs

@@ -0,0 +1,25 @@
+# Definition for singly-linked list.
+#
+# defmodule ListNode do
+#   @type t :: %__MODULE__{
+#           val: integer,
+#           next: ListNode.t() | nil
+#         }
+#   defstruct val: 0, next: nil
+# end
+
+defmodule Solution do
+  def reverse_list(head) when is_nil(head), do: nil
+
+  @spec reverse_list(head :: ListNode.t | nil) :: ListNode.t | nil
+  def reverse_list(head, arr \\ [])  do
+    new_arr = List.insert_at(arr, -1, head.val)
+    if head.next do
+      reverse_list(head.next, new_arr)
+    else
+      Enum.reduce(tl(new_arr), %ListNode{val: hd(new_arr), next: nil}, fn x, acc ->
+        %ListNode{val: x, next: acc}
+      end)
+    end
+  end
+end

valid-parentheses/han.exs

@@ -0,0 +1,21 @@
+defmodule Solution do
+  def is_valid(s) when byte_size(s) > 1 do
+    brackets = %{")" => "(", "}" => "{", "]" => "["}
+    opens = ["[", "{", "("]
+
+    result = Enum.reduce_while(String.graphemes(s), [], fn x, acc ->
+      cond do
+        x in opens ->
+          {:cont, List.insert_at(acc, -1, x)}
+        brackets[x] == List.last(acc) ->
+          {:cont, List.delete_at(acc, -1)}
+        true ->
+          {:halt, List.insert_at(acc, -1, x)}
+      end
+    end)
+
+    length(result) < 1
+  end
+
+  def is_valid(_), do: false
+end