236. Lowest Common Ancestor of a Binary Tree

Medium (Trung bình) Python 🔗 Xem trên LeetCode

📋 Đề Bài

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

 

Example 1:

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.

Example 2:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

Example 3:

Input: root = [1,2], p = 1, q = 2
Output: 1

 

Constraints:

  • The number of nodes in the tree is in the range [2, 105].
  • -109 <= Node.val <= 109
  • All Node.val are unique.
  • p != q
  • p and q will exist in the tree.

🧠 Thuật Toán & Kỹ Thuật

Tree Traversal (Duyệt cây)
⏱️ Thời gian O(n)
💾 Không gian O(1)

💻 Lời Giải

Python 0236-lowest-common-ancestor-of-a-binary-tree.py 
# 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':
        if root == p or root == q:
            return root
        
        nodeLeft, nodeRight = None, None
        
        if root.left:
            nodeLeft = self.lowestCommonAncestor(root.left, p, q)
            
        if root.right:
            nodeRight = self.lowestCommonAncestor(root.right, p, q)
            
        if nodeLeft and nodeRight:
            return root
        
        return nodeLeft or nodeRight