Interview Questions: Elementary Symbol Tables - Use Morris Tree Traversal

Author: proton

week4/elementary-symbol-tables/interview-questions/task3/TASK.md

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+### Inorder traversal with constant extra space
+
+Design an algorithm to perform an inorder traversal of a binary search tree using only a constant amount of extra space.

week4/elementary-symbol-tables/interview-questions/task3/solution.rb

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+class TreeNode
+  attr_accessor :value, :left, :right
+
+  def initialize(value = 0, left = nil, right = nil)
+    @value = value
+    @left = left
+    @right = right
+  end
+
+  def to_a
+    arr = []
+    arr += left.to_a if left
+    arr << value
+    arr += right.to_a if right
+    arr
+  end
+end
+
+def build_tree(array)
+  root = TreeNode.new(array[0])
+  queue = [root]
+  i = 1
+
+  while i < array.size
+    current = queue.shift
+
+    if array[i]
+      current.left = TreeNode.new(array[i])
+      queue << current.left
+    end
+    i += 1
+
+    if array[i]
+      current.right = TreeNode.new(array[i])
+      queue << current.right
+    end
+    i += 1
+  end
+
+  root
+end
+
+# Morris traversal
+
+tree = build_tree([4, 2, 6, 1, 3, 5, 7])
+
+ # Use Morris Tree Traversal.
+def morris_traversal(current_root)
+  while current_root
+    if current_root.left
+      current = current_root.left
+      current = current.right while current.right
+      current.right = current_root
+
+      temp = current_root
+      current_root = current_root.left
+      temp.left = nil
+    else
+      yield current_root.value
+      current_root = current_root.right
+    end
+  end
+end
+
+arr = []
+morris_traversal(tree) { |value| arr << value }
+fail unless arr == arr.sort
+p arr