MediumNo. 0971
Flip Binary Tree To Match Preorder Traversal — LeetCode 971 Python Solution
- Pattern
- Tree Traversal
- Topics
- Tree, Depth-First Search, Binary Tree
- Reading time
- 5 min
- Source
- leetcode.com
The problem
You are given the root of a binary tree with n nodes, where each node is uniquely assigned a value from 1 to n. You are also given a sequence of n values voyage, which is the desired pre-order traversal of the binary tree.
Example
- Input
- root = [1,2], voyage = [2,1]
- Output
- [-1]
- Explanation
- It is impossible to flip the nodes such that the pre-order traversal matches voyage.
Python solution
# 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 flipMatchVoyage(self, root: Optional[TreeNode], voyage: List[int]) -> List[int]:
def dfs(root):
nonlocal i, ok
if root is None or not ok:
return
if root.val != voyage[i]:
ok = False
return
i += 1
if root.left is None or root.left.val == voyage[i]:
dfs(root.left)
dfs(root.right)
else:
ans.append(root.val)
dfs(root.right)
dfs(root.left)
ans = []
i = 0
ok = True
dfs(root)
return ans if ok else [-1]Complexity
- Time
- O(n)
- Space · auxiliary
- O(n)where n is the number of nodes in the tree
Pattern: Tree Traversal
Choose the order — preorder, inorder, postorder, level — and the problem solves itself. LeetCode 971. Flip Binary Tree To Match Preorder Traversal is filed here because LeetCode tags it Tree and Binary Tree, which is the vocabulary this hub collects.
The tree traversal guide has the Python template for the pattern and the 225 LeetCode problems that use it.
Related problems
| No. | Problem | Difficulty |
|---|---|---|
| 0965 | Univalued Binary Tree | Easy |
| 0979 | Distribute Coins in Binary Tree | Medium |
| 0098 | Validate Binary Search Tree | Medium |
| 0099 | Recover Binary Search Tree | Medium |
| 0100 | Same Tree | Easy |
| 0101 | Symmetric Tree | Easy |
Frequently asked questions
- How hard is LeetCode 971. Flip Binary Tree To Match Preorder Traversal?
LeetCode 971. Flip Binary Tree To Match Preorder Traversal is rated Medium on LeetCode.
- What is the time complexity of LeetCode 971. Flip Binary Tree To Match Preorder Traversal?
The Python solution on this page runs in O(n).
- What is the space complexity of LeetCode 971. Flip Binary Tree To Match Preorder Traversal?
The Python solution on this page uses O(n), where n is the number of nodes in the tree auxiliary space.
- What topics does LeetCode 971. Flip Binary Tree To Match Preorder Traversal cover?
LeetCode 971. Flip Binary Tree To Match Preorder Traversal is tagged Tree, Depth-First Search and Binary Tree on LeetCode.