236.py

'''
	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 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).”

	Given the following binary search tree:  root = [3,5,1,6,2,0,8,null,null,7,4]

	        _______3______
	       /              \
	    ___5__          ___1__
	   /      \        /      \
	   6      _2       0       8
	         /  \
	         7   4
	Example 1:

	Input: root, p = 5, q = 1
	Output: 3
	Explanation: The LCA of of nodes 5 and 1 is 3.
'''

# 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:
        	return None

        if root == p or root == q:
        	return root

        l = self.lowestCommonAncestor(root.left, p, q)
        r = self.lowestCommonAncestor(root.right, p, q)

        if l and r:
        	return root
        return l if l else r