Leetcode: Lowest Common Ancestor of a Binary Search Tree

Get Idea from 西施豆腐渣 csdn .

Question

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

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

[code]    _______6______
   /              \
___2__          ___8__

/ \ / \

0 _4 7 9

/ \

3 5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

Analysis

Should take advantage of property of BST.

Solution

[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 root==None or p.val==None or q.val==None:
            return None

        if p.val>root.val and q.val>root.val:
            return self.lowestCommonAncestor(root.right, p, q)
        elif p.val<root.val and q.val<root.val:
            return self.lowestCommonAncestor(root.left, p, q)
        else:
            return root.val