LeetCode 235. Lowest Common Ancestor of a Binary Search Tree

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.
思路：二叉搜索树，假定r是p,q的公共祖先，则必定有r对应的节点值在p,q之间，反之，若r大于p且大于q，则公共祖先一定在左子树，小于类同。
AC代码如下：

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