LC 0114 [M] Flatten Binary Tree to Linked List - ALawliet/algorithms GitHub Wiki

preorder is: root - left - right

so this reverse preorder: right - left - root

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def flatten(self, root: Optional[TreeNode]) -> None:
        """
        Do not return anything, modify root in-place instead.
        """
    def __init__(self):
        self.prev = None

    def flatten(self, root):
        if not root: return None
        
        # right
        self.flatten(root.right)

        # left
        self.flatten(root.left)

        # root
        root.right = self.prev
        root.left = None
        self.prev = root

iterative

class Solution:
    def flatten(self, root: TreeNode) -> None:
        curr = root
        
        while curr:
            if curr.left != None:
                p = curr.left
                while p.right != None:
                    p = p.right
                    
                p.right = curr.right
                
                curr.right = curr.left
                curr.left = None
            
            curr = curr.right