验证二叉查找树（LintCode）

 1 /**
 2  * Definition of TreeNode:
 3  * public class TreeNode {
 4  *     public int val;
 5  *     public TreeNode left, right;
 6  *     public TreeNode(int val) {
 7  *         this.val = val;
 8  *         this.left = this.right = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     /**
14      * @param root: The root of binary tree.
15      * @return: True if the binary tree is BST, or false
16      */
17      
18     public boolean isValidBST(TreeNode root) {
19         List<Integer> list = new ArrayList<Integer>();
20         midOrder(root,list);
21         for(int i=1;i<list.size();i++) {
22             if(list.get(i-1) >= list.get(i))return false;
23         }
24         
25         return true;
26     }
27     
28     public void midOrder(TreeNode root,List<Integer> list) {
29         if(root == null) return;
30         midOrder(root.left,list);
31         list.add(root.val);
32         midOrder(root.right,list);
33     }
34 }