leetcode 235:Lowest Common Ancestor of a Binary Search Tree

题目描述：

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.

题目大意：找出两个节点的最低公共祖先（注意：一个节点可以看做自己的后代节点）

解题思路：

给出当前遍历到的结点r,找出结点p,q的最低公共祖先。

1.如果两个结点分别位于r节点两侧，则根节点为最小公共祖先；

如果r节点小于p节点的值，则在r结点的左子树中继续寻找；否则在r结点的右子树中寻找；

代码实现：

/**
* 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((p.val - root.val ) * (q.val - root.val) <=0)
return root;
else if(p.val > root.val)
return  lowestCommonAncestor(root.right,p,q);
else
return lowestCommonAncestor(root.left,p,q);
}
}