題目：Validate Binary Search Tree

難度：medium

問題描述：

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater than
  the node's key.
• Both the left and right subtrees must also be binary search trees.

Example 1:

    2
   / \
  1   3

Example 2:

    1
   / \
  2   3

檢驗一棵樹是不是二叉搜尋樹。中序遍歷整棵樹，檢驗輸出元素是不是有序的即可。

具體程式碼如下：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        if(root==null||root.left==null&&root.right==null){
			return true;
		}
		ArrayList<Integer> list=new ArrayList<Integer>();
		zhongxubianli(root,list);
		
		int temp=0,w=0;
		Iterator<Integer> iter=list.iterator();
		if(iter.hasNext()){
			temp=iter.next();
		}
		while(iter.hasNext()){
			w=iter.next();
			if(w<=temp){
				return false;
			}
			temp=w;
		}
		return true;
    }
    public void zhongxubianli(TreeNode root,ArrayList<Integer> list){
		if(root.left!=null){
			zhongxubianli(root.left,list);
		}
		list.add(root.val);
		if(root.right!=null){
			zhongxubianli(root.right,list);
		}
	}
}