【leetcode】235. Lowest Common Ancestor of a Binary Search Tree

题目要求：

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.

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if(root==null)
{
return null;
}
//二者中有一个是根节点就返回根节点
if(p.val==root.val||q.val==root.val)
{
return root;
}
//分别在树的左右两边就返回根节点
if((p.val<root.val)&&(q.val>root.val))
{
return root;
}
if((p.val>root.val)&&(q.val<root.val))
{
return root;
}
//都在左边，就对左子树递归调用此函数
if((p.val<root.val)&&(q.val<root.val))
{
return lowestCommonAncestor(root.left,p,q);
}
//都在右边，就对右子树递归调用此函数
if((p.val>root.val)&&(q.val>root.val))
{
return lowestCommonAncestor(root.right,p,q);
}
return null;
}
}