41.Validate Binary Search Tree（判断是否为二叉搜索树）

Level：

  Medium

题目描述：

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:

Input:
    2
   / \
  1   3
Output: true

Example 2:

    5
   / \
  1   4
     / \
    3   6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value is 5 but its right child's value is 4.

思路分析：

​  验证一棵二叉树是不是二叉搜索树有两种方法，一种是按照它的性质进行验证，那就是左<根<右，一种是采用中序遍历的方法看它是不是有序的。这里我们采用第一种方法。

代码：

public class Solution{
    public boolean isValidBST(TreeNode root){
        if(root==null)
            return true;
        return isBST(root,null,null);
    }
    public boolean isBST(TreeNode root,Integer low ,Integer high){
        if(root==null)
            return true;
        if(low!=null&&root.val<=low||high!=null&&root.val>=high)
            return false;
        return isBST(root.left,low,root.val)&&isBST(root.right,root.val,high);
    }
}