### 问题描述

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.

### 问题分析

根节点root，寻找节点A和B的最低公共祖先节点：



### 代码

class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
if(root==0) return 0;
if(p==0||q==0) return root;
int val = root->val;
if(p->val==val) return p;
if(q->val==val) return q;
if(p->val<val&&q->val>val||p->val>val&&q->val<val) {
return root;
}
if(p->val<val&&q->val<val) return lowestCommonAncestor(root->left, p, q);
if(p->val>val&&q->val>val) return lowestCommonAncestor(root->right, p, q);
}
};