In a given Binary Tree (BT), convert it to a Doubly Linked List(DLL). The left and right pointers in nodes are to be used as previous and next pointers respectively in converted DLL. The order of nodes in DLL must be same as Inorder of the given Binary Tree. The first node of Inorder traversal (left most node in BT) must be head node of the DLL.

class Node(object):
#Binary tree Node class has data, left and right child
def __init__(self, item):
self.data = item
self.left = None
self.right = None
def BTToDLLUtil(root):
#This is a utility function to
#convert the binary tree to doubly
#linked list. Most of the core task
#is done by this function.
if root is None:
return root
# Convert left subtree
# and link to root
if root.left:
# Convert the left subtree
left = BTToDLLUtil(root.left)
# Find inorder predecessor, After
# this loop, left will point to the
# inorder predecessor of root
while left.right:
left = left.right
# Make root as next of predecessor
left.right = root
# Make predecessor as
# previous of root
root.left = left
# Convert the right subtree
# and link to root
if root.right:
# Convert the right subtree
right = BTToDLLUtil(root.right)
# Find inorder successor, After
# this loop, right will point to
# the inorder successor of root
while right.left:
right = right.left
# Make root as previous
# of successor
right.left = root
# Make successor as
# next of root
root.right = right
return root
def BTToDLL(root):
if root is None:
return root
# Convert to doubly linked
# list using BLLToDLLUtil
root = BTToDLLUtil(root)
# We need pointer to left most
# node which is head of the
# constructed Doubly Linked list
while root.left:
root = root.left
return root
def print_list(head):
"""Function to print the given
doubly linked list"""
if head is None:
return
while head:
print(head.data, end = " ")
head = head.right
# Driver Code
if __name__ == '__main__':
root = Node(10)
root.left = Node(12)
root.right = Node(15)
root.left.left = Node(25)
root.left.right = Node(30)
root.right.left = Node(36)
head = BTToDLL(root)
print_list(head)

Output:

25 12 30 10 36 15

>>>

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