41.Validate Binary Search Tree (to determine whether the binary search tree)

Level：

  Medium

Subject description:

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.

Analysis of ideas:

Verify binary tree is not binary search tree, there are two methods, one is to be verified according to its nature, that is, the left <root <right one is using the method preorder see if it is ordered . Here we use the first method.

Code:

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);
    }
}