Validate Binary Search Tree

Source

• lintcode: Validate Binary Search Tree

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
An example:

  2
 / \
1   4
   / \
  3   5
The above binary tree is serialized as {2,1,4,#,#,3,5} (in level order).

Java

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: True if the binary tree is BST, or false
     */
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, null, null);
    }
    public boolean isValidBST(TreeNode root, TreeNode min, TreeNode max){
        if(root==null) return true;
        if((max!=null && root.val>=max.val) || (min!=null && root.val<=min.val)) return false;
        return isValidBST(root.left, min, root) && isValidBST(root.right, root, max);
    }
}