Likou 236. The nearest common ancestor of a binary tree (python implementation)

Topic source: 236. The nearest common ancestor of a binary tree
Topic: Given a binary tree, find the nearest common ancestor of two specified nodes in the tree.

The definition of the nearest common ancestor in Baidu Encyclopedia is: "For two nodes p and q of a rooted tree T, the nearest common ancestor is represented as a node x, which satisfies that x is the ancestor of p, q and the depth of x is as large as possible (a A node can also be its own ancestor)."


Idea: Recursion

1. If the root node is empty, or if the root node is equal to p or q, then the root node is its nearest common ancestor.
2. If the root node is not empty, recursively call the left and right subtrees of the root node
3. If the left subtree is empty, indicating that p and q are not in the left subtree, return the right subtree.
4. If the right subtree is empty, indicating that p and q are not in the right subtree, return the left subtree.
5. If the left and right subtrees are not empty, it means that the nearest common node is the root node, and the root node is returned.

Explanation: In fact, it is to use the idea of ​​recursion to search for p and q left and right, and return the node that first encounters the p and q nodes, and then judge whether p and q are in the same subtree, and if so, return the corresponding subtree, If not, the common ancestor must be the root node, which is always recursively judged.
python code:

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def lowestCommonAncestor(self, root, p, q):
        """
        :type root: TreeNode
        :type p: TreeNode
        :type q: TreeNode
        :rtype: TreeNode
        """
        if not root or (root==p or root==q):
            return root
        left = self.lowestCommonAncestor(root.left,p,q)
        right = self.lowestCommonAncestor(root.right,p,q)
        if not left:
            return right
        if not right:
            return left
        return root

Reference source: [leetcode] The sword refers to Offer 68 - II. The nearest common ancestor of a binary tree (python3 version) The

nearest common ancestor of a binary tree (Python version)