检查是否为BST

请实现一个函数，检查一棵二叉树是否为二叉查找树。

给定树的根结点指针TreeNode* root，请返回一个bool，代表该树是否为二叉查找树

代码如下：

package com.mianshi;

import java.util.Stack;

public class jingdian_21 {

	static class TreeNode {
	    int val = 0;
	    TreeNode left = null;
	    TreeNode right = null;
	    public TreeNode(int val) {
	        this.val = val;
	    }
	}
	    
	public static void main(String[] args) {
		TreeNode l1=new TreeNode(4);
		TreeNode l2=new TreeNode(2);
		TreeNode l3=new TreeNode(6);
		TreeNode l4=new TreeNode(1);
		TreeNode l5=new TreeNode(3);
		TreeNode l6=new TreeNode(5);
		TreeNode l7=new TreeNode(7);
		l1.left=l2;
		l1.right=l3;
		l2.left=l4;
		l2.right=l5;
		l3.left=l6;
	  	l3.right=l7;
        System.out.print(jingdian_21.isValidBST(l1));
	}
	public static boolean isValidBST(TreeNode root) {
        if(root==null) return true;
        Stack<TreeNode> stack =new Stack<TreeNode>();
        int pre=0;
        TreeNode p=root;
        boolean isFirst=true;
        while(p!=null || !stack.isEmpty()){
        	  while(p!=null){
        		  stack.push(p);
        		  p=p.left;
        	  }
        	  p=stack.pop();
        	  if(isFirst){
        		  pre=p.val;
        		  isFirst=false;
        	  }else if(pre>p.val){
        		  return false;
        	  }else{
        		  pre=p.val;
        	  }
        	  p=p.right;
        }
        return true;
	}
}