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Author: pushpit-J19

Data Structures/Trees/binary_tree_to_dll.py

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+"""
+Binary trees are a type of data tree data structure in which a node can only have 0,1 or 2 children only.
+Linked list is a type of linear data structure in which one object/node along with data, also contains the address of next object/node.
+Doubly linked list is a type of linked list in which a node points to both the element before it, and after it.
+
+A binary tree can be converted to a doubly linked list based on in order traversal, where left and right pointers point to prev and next element Eg :-
+
+     10 
+    /  \
+   12   15
+   /\    /
+ 25 30  36
+
+can be converted to the doubly linked list
+
+25 <-> 12 <-> 30 <-> 10 <-> 36 <-> 15
+
+We have used an iterative approach here, keeping the track of head and tail of the doubly linked list,
+recursively solving left subtree, then redirecting the pointers to connect the last element and the current element,
+and finally recursively solving the right subtree
+
+"""
+
+
+
+
+class Node :
+    """Class, to define node of the binary tree and dll, has data, left, and right as attributes"""
+    def  __init__(self, value):
+        self.data = value
+        self.left = None
+        self.right = None
+
+def binary_tree_to_dll(node, head, tail):
+    """This function converts a binary tree to a doubly linked list with recursive approach
+       input: tree root or node, head and tail pointer of DLL
+       returns : the head and tail of the the linked lists 
+    """
+
+    if node == None :
+        return head, tail
+    
+    head, tail = binary_tree_to_dll(node.left, head, tail)  # converting the left subtree
+
+    # updating the tail of the list to point towards current node
+    if head == None:
+        head = node
+    else :
+        tail.right = node
+        node.left = tail
+    tail = node         # shifting the tail to the latest node
+
+    head, tail = binary_tree_to_dll(node.right, head, tail)  # converting the right subtree
+
+    return head, tail
+
+def print_list (head):
+    """ iterates over the linked list prints the data elements
+        input : head of the linked list
+        prints the elements, does not return anything
+    """
+    while head != None:
+        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, tail = None, None
+    head, tail = binary_tree_to_dll(root, head, tail)
+    print("\nEquivaltent doubly linked list : ", end="")
+    print_list (head)
+    print("\n")
+
+
+
+    # Extra examples for testing
+    """
+    root = Node(-5)
+    root.left = Node(10)
+    root.right = Node(2) 
+
+    # ans : 10 -5 2
+    """
+
+    """
+    root = Node(3)
+    root.left = Node(5)
+    root.right = Node (7)
+    root.left.left = Node(9)
+    root.left.right = Node(11)
+    root.right.left = Node(13)
+    root.right.right = Node(15)
+    root.left.left.left = Node(17)
+
+    # ans : 17 9 5 11 3 13 7 15
+    """