Increasing Order Search Tree.py

'''
Given a tree, rearrange the tree in in-order so that the leftmost node in the tree is now the root of the tree, and every node has no left child and only 1 right child.

Example 1:
Input: [5,3,6,2,4,null,8,1,null,null,null,7,9]

       5
      / \
    3    6
   / \    \
  2   4    8
 /        / \ 
1        7   9

Output: [1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]

 1
  \
   2
    \
     3
      \
       4
        \
         5
          \
           6
            \
             7
              \
               8
                \
                 9  

Note:

    The number of nodes in the given tree will be between 1 and 100.
    Each node will have a unique integer value from 0 to 1000.


'''

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def increasingBST(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        if not root:
            return None
        self.head = None
        self.pre = None
        self.build(root)
        return self.head
    
    def build(self, node):
        if node.left:
            self.build(node.left)
        if self.pre:
            self.pre.right = node
        node.left = None
        self.pre = node
        if not self.head:
            self.head = node
        if node.right:
            self.build(node.right)