LintCode Validate Binary Search Tree

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 thanthe 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

 /**
  * 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) {
             return true;
         }
         if ((root.left == null) && (root.right == null)) {
             return true;
         }

         boolean left = false;
         boolean right = false;
         if (root.left == null) {
             left = true;
         }
         else if ((root.left != null) && (root.left.val < root.val)) {
             left = isValidBST(root.left);
         }else {
             return false;
         }

         if (root.right == null) {
             right = true;
         }
         else if ((root.right != null) && (root.right.val > root.val)) {
             right = isValidBST(root.right);
         }else {
             return false;
         }

         return left && right;
     }
 }