LeetCode *** 235. Lowest Common Ancestor of a Binary Search Tree

题目：

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.

分析：

嗯。。我一开始以为LCA的意思是数字最小。。结果才发现是离得最近的。。那么没啥意思了。。。只要p和q一个在root的左边另外一个在右边，那么root就是所求结果了。。。

代码：

/**
* 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;
TreeNode* res=root;

while(res){
if((res->val<=p->val&&res->val>=q->val)||(res->val>=p->val&&res->val<=q->val))return res;
else if(res->val>p->val&&res->val>q->val)res=res->left;
else if(res->val<p->val&&res->val<q->val)res=res->right;
}
}
};