一.解法

https://leetcode-cn.com/problems/lowest-common-ancestor-of-a-binary-search-tree/
要点：递归，树
Python，C++，Java都用了递归的方法。
由于是二叉搜索树，root左边所有的值都小于root的值，root右边所有的值都大于root的值。
所以如果p,q的值都小于root的值，就去root.left找，
如果p,q的值都大于root的值，就去root.right找
否则说明root就是最近公共祖先

二.Python实现

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
        
        if p.val<root.val and q.val<root.val:
            return self.lowestCommonAncestor(root.left,p,q) 
        elif p.val>root.val and q.val>root.val:
            return self.lowestCommonAncestor(root.right,p,q)    
        else:
            return root

三.C++实现

/**
 * 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 (p == root || q == root || p->val<root->val&&q->val>root->val || p->val > root->val&&q->val < root->val)
        return root;
    if (p->val < root->val&&q->val < root->val)
        return lowestCommonAncestor(root->left, p, q);
    return lowestCommonAncestor(root->right, p, q);
}
};

四.java实现

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(p.val<root.val&&q.val<root.val) return lowestCommonAncestor(root.left,p,q);
        else if(p.val>root.val&&q.val>root.val) return lowestCommonAncestor(root.right,p,q);
        else return root;
        
    }
}