MediumNo. 1644LeetCode Premium
Lowest Common Ancestor of a Binary Tree II — LeetCode 1644 Python Solution
- Pattern
- Tree Traversal
- Topics
- Tree, Depth-First Search, Binary Tree
- Reading time
- 5 min
- Source
- leetcode.com
The problem
Given the root of a binary tree, return the lowest common ancestor (LCA) of two given nodes, p and q. If either node p or q does not exist in the tree, return null.
Example
- Input
- root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
- Output
- 3
- Explanation
- The LCA of nodes 5 and 1 is 3.
Python solution
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def lowestCommonAncestor(
self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode'
) -> 'TreeNode':
def dfs(root, p, q):
if root is None:
return False
l = dfs(root.left, p, q)
r = dfs(root.right, p, q)
nonlocal ans
if l and r:
ans = root
if (l or r) and (root.val == p.val or root.val == q.val):
ans = root
return l or r or root.val == p.val or root.val == q.val
ans = None
dfs(root, p, q)
return ansComplexity
- Time
- O(V+E)
- Space · auxiliary
- O(V)
Pattern: Tree Traversal
Choose the order — preorder, inorder, postorder, level — and the problem solves itself. LeetCode 1644. Lowest Common Ancestor of a Binary Tree II 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 |
|---|---|---|
| 0236 | Lowest Common Ancestor of a Binary Tree | Medium |
| 0098 | Validate Binary Search Tree | Medium |
| 0099 | Recover Binary Search Tree | Medium |
| 0100 | Same Tree | Easy |
| 0101 | Symmetric Tree | Easy |
| 0104 | Maximum Depth of Binary Tree | Easy |
Frequently asked questions
- How hard is LeetCode 1644. Lowest Common Ancestor of a Binary Tree II?
LeetCode 1644. Lowest Common Ancestor of a Binary Tree II is rated Medium on LeetCode.
- What is the time complexity of LeetCode 1644. Lowest Common Ancestor of a Binary Tree II?
The Python solution on this page runs in O(V+E).
- What is the space complexity of LeetCode 1644. Lowest Common Ancestor of a Binary Tree II?
The Python solution on this page uses O(V) auxiliary space.
- What topics does LeetCode 1644. Lowest Common Ancestor of a Binary Tree II cover?
LeetCode 1644. Lowest Common Ancestor of a Binary Tree II is tagged Tree, Depth-First Search and Binary Tree on LeetCode.
- Is LeetCode 1644. Lowest Common Ancestor of a Binary Tree II a premium problem?
Yes. LeetCode 1644. Lowest Common Ancestor of a Binary Tree II is a LeetCode Premium problem, so the full statement and test cases require a paid LeetCode subscription.