Leetcode 之 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.

这里写到了说是BST，所以有特殊性，因为BST的左儿子一定比右儿子大。因此可以遍历树的时候根据节点与p,q节点val的大小关系决定遍历的方向（左边还是右边）。

而Lowest Common Ancestor 这个定义正好也是（node.val>=left.val&&node.val<=right.val）的node

代码如下：

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {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;        }