Validate Binary Search Tree(lintcode 95)

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. A single node tree is a BST

Example

An example:

    2          
   / \             
  1   4          
     / \          
    3   5

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

Interface

/**
 * 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) {
        // write your code here
    }
}

Solution

/**
 * 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
     */
    private int lastVal = Integer.MIN_VALUE;
    private boolean firstNode = true;

    public boolean isValidBST(TreeNode root) {
        // write your code here
        if (root == null) {
            return true;
        }

        if (!isValidBST(root.left)) {
            return false;
        }

        if (!firstNode && lastVal >= root.val) {
            return false;
        }

        firstNode = false;
        lastVal = root.val;

        if (!isValidBST(root.right)) {
            return false;
        }

        return true;
    }
}