LintCode : Validate Binary Search Tree

LintCode :  Validate Binary Search Tree

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).

 

/**
 * 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
        if (root != null) {
            if (root.left != null) {
                if (root.val <= root.left.val) {
                    return false;
                }
            }
            if (root.right != null) {
                if (root.val >= root.right.val) {
                    return false;
                }
            }
            return validBSTHelper(root.left, root.val, false) & validBSTHelper(root.right, root.val, true);
        }
        return true;
    }
    
    public boolean validBSTHelper(TreeNode root, int lastVal, boolean k) {
        if (root != null) {
            if (root.left != null) {
                if (root.val <= root.left.val || (root.left.val <= lastVal) == k) {
                    return false;
                }
            }
            if (root.right != null) {
                if (root.val >= root.right.val || (root.right.val <= lastVal) == k) {
                    return false;
                }
            }
            return validBSTHelper(root.left, root.val, false) & validBSTHelper(root.right, root.val, true);
        }
        return true;
    }
}

The most important thing here is to remember that for one node, all the values in the left are less, all the values in the right are bigger.