1534.convert-binary-search-tree-to-sorted-doubly-linked-list.py

#  Tag: Binary Search Tree, Linked List, Binary Tree, Divide and Conquer
#  Time: O(N)
#  Space: O(H)
#  Ref: -
#  Note: BST | Leetcode-426

#  Convert a BST to a sorted circular doubly-linked list in-place.
#  Think of the left and right pointers as synonymous to the previous and next pointers in a doubly-linked list.
#  
#  Let's take the following BST as an example, it may help you understand the problem better:
#  
#  ![bstdlloriginalbst](<https://assets.leetcode.com/uploads/2018/10/12/bstdlloriginalbst.png>)
#  
#  We want to transform this BST into a circular doubly linked list.
#  Each node in a doubly linked list has a predecessor and successor.
#  For a circular doubly linked list, the predecessor of the first element is the last element, and the successor of the last element is the first element.
#  
#  The figure below shows the circular doubly linked list for the BST above.
#  The "head" symbol means the node it points to is the smallest element of the linked list.
#  
#  ![bstdllreturndll](<https://assets.leetcode.com/uploads/2018/10/12/bstdllreturndll.png>)
#  
#  Specifically, we want to do the transformation in place.
#  After the transformation, the left pointer of the tree node should point to its predecessor, and the right pointer should point to its successor.
#  We should return the pointer to the first element of the linked list.
#  
#  The figure below shows the transformed BST.
#  The solid line indicates the successor relationship, while the dashed line means the predecessor relationship.
#  
#  ![bstdllreturnbst](<https://assets.leetcode.com/uploads/2018/10/12/bstdllreturnbst.png>)
#  
#  **Example 1:**
#  ```
#  Input: {4,2,5,1,3}
#          4
#         /  \
#        2   5
#       / \
#      1   3
#  Output: "left:1->5->4->3->2  right:1->2->3->4->5"
#  Explanation:
#  Left: reverse output
#  Right: positive sequence output
#  ```
#  **Example 2:**
#  ```
#  Input: {2,1,3}
#          2
#         /  \
#        1   3
#  Output: "left:1->3->2  right:1->2->3"
#  ```
#  
#  

"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None
"""

class Solution:
    """
    @param root: root of a tree
    @return: head node of a doubly linked list
    """
    def treeToDoublyList(self, root):
        # Write your code here.

        node = root
        stack = []
        head = None
        last = None

        while len(stack) > 0 or node:
            while node:
                stack.append(node)
                node = node.left

            node = stack.pop()
            if head is None:
                head = node

            if last:
                last.right = node
                node.left = last
            last = node

            node = node.right

        head.left = last
        last.right = head
        return head