Depth First Binary Tree Search Using Recursion in Python

When it is required to perform depth first search on a tree using recursion, a class is defined, and methods are defined on it that help perform breadth first search.

Below is a demonstration for the same −

Example

 Live Demo

class BinaryTree_struct:
   def __init__(self, key=None):
      self.key = key
      self.left = None
      self.right = None

   def set_root(self, key):
      self.key = key

   def insert_at_left(self, new_node):
      self.left = new_node

   def insert_at_right(self, new_node):
      self.right = new_node

   def search_elem(self, key):
      if self.key == key:
         return self
      if self.left is not None:
         temp = self.left.search(key)
         if temp is not None:
            return temp
      if self.right is not None:
         temp = self.right.search(key)
         return temp
      return None

   def depth_first_search(self):
      print('entering {}...'.format(self.key))
      if self.left is not None:
         self.left.depth_first_search()
      print('at {}...'.format(self.key))
      if self.right is not None:
         self.right.depth_first_search()
      print('leaving {}...'.format(self.key))

btree_instance = None

print('Menu (no duplicate keys)')
print('insert <data> at root')
print('insert <data> left of <data>')
print('insert <data> right of <data>')
print('dfs')
print('quit')

while True:
   my_input = input('What would you like to do? ').split()

   op = my_input[0].strip().lower()
   if op == 'insert':
      data = int(my_input[1])
      new_node = BinaryTree_struct(data)
      sub_op = my_input[2].strip().lower()
      if sub_op == 'at':
         btree_instance = new_node
      else:
         position = my_input[4].strip().lower()
         key = int(position)
         ref_node = None
         if btree_instance is not None:
            ref_node = btree_instance.search_elem(key)
         if ref_node is None:
            print('No such key.')
            continue
         if sub_op == 'left':
            ref_node.insert_at_left(new_node)
         elif sub_op == 'right':
            ref_node.insert_at_right(new_node)
   elif op == 'dfs':
      print('depth-first search traversal:')
      if btree_instance is not None:
         btree_instance.depth_first_search()
      print()

   elif op == 'quit':
      break

Output

Menu (no duplicate keys)
insert <data> at root
insert <data> left of <data>
insert <data> right of <data>
dfs
quit
What would you like to do? insert 5 at root
What would you like to do? insert 6 left of 5
What would you like to do? insert 8 right of 5
What would you like to do? dfs
depth-first search traversal:
entering 5...
entering 6...
at 6...
leaving 6...
at 5...
entering 8...
at 8...
leaving 8...
leaving 5...
What would you like to do? quit
Use quit() or Ctrl-D (i.e. EOF) to exit

Explanation

• The ‘BinaryTree_struct’ class with required attributes is created.

• It has an ‘init’ function that is used to assign ‘left’ and ‘right’ nodes to ‘None’.

• Another method named ‘set_root’ is defined to specify the root of the tree.

• Another method named ‘insert_at_left’ is defined that helps add nodes to the left of the tree.

• Another method named ‘insert_at_right’ is defined that helps add nodes to the right of the tree.

• Another method named ‘search_elem’ is defined that helps search for a specific element.

• A method named ‘depth_first_search’ is defined, that helps perform depth first search on the binary tree.

• An instance of the class is created and assigned to ‘None’.

• A menu is given.

• The user input is taken for the operation that needs to be performed.

• Depending on the user’ choice, the operation is performed.

• Relevant output is displayed on the console.