給定一個二叉搜尋樹, 找到該樹中兩個指定節點的最近公共祖先。

這道題一定要了解二叉搜尋樹的性質，右邊比左邊大。

python：

# 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:
            return None
        if (p.val <= root.val and root.val <= q.val) or (q.val <= root.val and root.val <= p.val):
            return root
        if (p.val < root.val) and (q.val < root.val):
            return self.lowestCommonAncestor(root.left,p,q)
        if (p.val > root.val) and (q.val > root.val):
            return self.lowestCommonAncestor(root.right,p,q)
 

C++： 

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if(root == NULL) return NULL;
        if((root->val>=p->val&&root->val<=q->val) || (root->val>=q->val&&root->val<=p->val)) return root;
        if(root->val>p->val&&root->val>q->val) return lowestCommonAncestor(root->left,p,q);
        if(root->val<p->val&&root->val<q->val) return lowestCommonAncestor(root->right,p,q);
        return NULL;
    }
};