Lowest common ancestor of a binary tree(Python)
2051 ワード
Problem: 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).”
Given the following binary tree: root = [3,5,1,6,2,0,8,null,null,7,4]
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.
Note:
All of the nodes’ values will be unique. p and q are different and both values will exist in the binary tree.
Resource:LeetCode Link:https://leetcode-cn.com/problems/lowest-common-ancestor-of-a-binary-tree
Analyse: First,I desided to use the method of recursion. Second,I cross the river by feeling for stones,namely that I tried to write a piece of code. Third,I draw a draft like this to check the correctness of my code.
Code:
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).”
Given the following binary tree: root = [3,5,1,6,2,0,8,null,null,7,4]
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.
Note:
All of the nodes’ values will be unique. p and q are different and both values will exist in the binary tree.
Resource:LeetCode Link:https://leetcode-cn.com/problems/lowest-common-ancestor-of-a-binary-tree
Analyse: First,I desided to use the method of recursion. Second,I cross the river by feeling for stones,namely that I tried to write a piece of code. Third,I draw a draft like this to check the correctness of my code.
Code:
class Solution(object):
def lowestCommonAncestor(self, root, p, q):
"""
:type root: TreeNode
:type p: TreeNode
:type q: TreeNode
:rtype: TreeNode
"""
self.p=p
self.q=q
self.res=None
self.recursion(root)
return self.res
def recursion(self,root):
if root==None:
return False
left=self.recursion(root.left)
right=self.recursion(root.right)
if ((left==True)+(right==True)+((root==self.p) or (root==self.q)))==2:
self.res=root
return True
if root==self.p or root==self.q:
return True
if left==True or right==True:
return True
return False