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.

公共祖先要么在左子树，要么在右子树，要么就在根节点，采用递归算法，代码如下：

/**

 * 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.val>Math.max(p.val,q.val)){

            return lowestCommonAncestor(root.left,p,q);

        }

         else if(root.val<Math.min(p.val,q.val)){

            return lowestCommonAncestor(root.right,p,q);

        }else{

            return root;

        }

    }

}