235. Lowest Common Ancestor of a Binary Search Tree

题目：

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.

链接：　　http://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/

注意第4，5行的判断不要错

public class Solution {
    public 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.right, p, q);
        else if (root.val > p.val && root.val > q.val) return lowestCommonAncestor(root.left, p, q);
        else return root;
    }
}