235. Lowest Common Ancestor of a Binary Search Tree

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).”

        _______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.

这个题目首先要看清是二查搜索树，我当时忘记这一点了所以走了很多弯路，

这道题因为二查搜索树的缘故简单很多，：

1. 如果root的值在pq之间，则root就是lca

2.如果pq 的值都小于root，那么pq就在root的左子树上，自然他们的lca也在root的左子树上

3.如果pq的值都大于root，那么pq就在root的右子树上，自然他们的lca也在root的右子树上

所以最后的程序是：

class Solution(object):    def lowestCommonAncestor(self, root, p, q):        """        :type root: TreeNode        :type p: TreeNode        :type q: TreeNode        :rtype: TreeNode        """        if p.val == q.val:        return p        elif p.val >q.val:        maxn = p        minn = q        else :        maxn = q        minn = p        if maxn.val >root.val:        if minn.val <=root.val:        return root        else :        return self.lowestCommonAncestor(root.right, p, q)        elif maxn.val == root.val:        return root        else :        return self.lowestCommonAncestor(root.left, p, q)