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114. Flatten Binary Tree To Linked List

Linked List Stack Tree Depth-First Search Binary Tree

Problem - Flatten Binary Tree To Linked List

Medium

Given the root of a binary tree, flatten the tree into a "linked list":

  • The "linked list" should use the same TreeNode class where the right child pointer points to the next node in the list and the left child pointer is always null.
  • The "linked list" should be in the same order as a pre-order traversal of the binary tree.

 

Example 1:

Input: root = [1,2,5,3,4,null,6]
Output: [1,null,2,null,3,null,4,null,5,null,6]

Example 2:

Input: root = []
Output: []

Example 3:

Input: root = [0]
Output: [0]

 

Constraints:

  • The number of nodes in the tree is in the range [0, 2000].
  • -100 <= Node.val <= 100

 

Follow up: Can you flatten the tree in-place (with O(1) extra space)?

Solutions

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# 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.
        """
        while root:
            if root.left:
                last = root.left
                while last.right:
                    last = last.right

                last.right = root.right
                root.right = root.left
                root.left = None
            root = root.right

Submission Stats:

  • Runtime: 0 ms (100.00%)
  • Memory: 18.2 MB (26.50%)