二叉树---检查是否为BST

import java.util.*;

/*
public class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}*/
public class Checker {
public boolean checkBST(TreeNode root) {
// write code here

if(root==null)
return true;

if(root.left!=null)
{
if(root.left.val>root.val)
return false;
if(root.left.right!=null&&root.left.right.val>root.val)
return false;
}

if(root.right!=null)
{
if(root.right.val<root.val)
return false;
if(root.right.left!=null&&root.right.left.val<root.val)
return false;
}

return checkBST(root.left)&&checkBST(root.right);

}
}