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 1:

  1.     2
  2.    / \
  3.   1   3

Binary tree [2,1,3], return true.

Example 2:

  1.     1
  2.    / \
  3.   2   3

Binary tree [1,2,3], return false.

Solution:

  1. /**
  2.  * Definition for a binary tree node.
  3.  * public class TreeNode {
  4.  *     int val;
  5.  *     TreeNode left;
  6.  *     TreeNode right;
  7.  *     TreeNode(int x) { val = x; }
  8.  * }
  9.  */
  10. public class Solution {
  11.   public boolean isValidBST(TreeNode root) {
  12.     return isValidBST(root, null, null);
  13.   }
  15.   boolean isValidBST(TreeNode root, Integer min, Integer max) {
  16.     if (root == null)
  17.       return true;
  19.     if (min != null && root.val <= min)
  20.       return false;
  22.     if (max != null && root.val >= max)
  23.       return false;
  25.     return isValidBST(root.left, min, root.val) && isValidBST(root.right, root.val, max);
  26.   }
  27. }