python --- LeetCode之 236. Lowest Common Ancestor of a Binary Tree

题目：
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).”
题目理解：
题目就是要找到二叉树中两个节点的最近公共节点。
方法：
递归，用一个辅助函数以此节点为根结点找p和q，如果在左子树找到p(q)，右子树找到q（p）,则此节点即为所求，如果在左子树（右子树）找到p和q，则左子树（右子树）停止寻找，返回该节点的右节点（左节点），在右子树（左子树）继续寻找，直到p和q分别在左子树和右子树，则此时的节点即为所求。
代码：

class Solution(object):
    def lowestCommonAncestor(self, root, p, q):
        """
        :type root: TreeNode
        :type p: TreeNode
        :type q: TreeNode
        :rtype: TreeNode
        """
        return self.heaper(root,p,q)
    def heaper(self, node, p, q):
        if node == None or node ==p or node == q:
            return node
        left = self.heaper(node.left,p,q)   # 如果左子树找到目标节点，则返回左节点，否则就是None
        right = self.heaper(node.right,p,q) # 如果右子树找到目标节点，则返回右节点，否则就是None
        if left and right:
            return node
        if left and not right:   # if left is not None and right is None:
            return left
        if not left and right:
            return right