[leetcode] 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.

这道题是给出二叉搜索树中两个节点，找他们的最低祖先节点，题目难度为Easy。

根据题目中LCA的定义和二叉搜索树的特点，我们可以遍历二叉搜索树，如果当前节点值比两个节点值都大，说明LCA在左子树中；如果当前节点值比两个节点值都小，说明LCA在右子树中；如果介于两者之间，说明当前节点即是LCA。题目比较简单，就不推荐递归的做法了。具体代码：

class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
TreeNode* node = root;
if(!root || !p || !q) return NULL;
int maxVal = max(p->val, q->val);
int minVal = min(p->val, q->val);
while(node) {
if(node->val > maxVal) node = node->left;
else if(node->val < minVal) node = node->right;
else return node;
}
}
};