Leetcode–Validate Binary Search Tree

The Problem:

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.


  1. Note that a valid BST should have ALL node in the left less than the root (of each level), not only the immediate left child. This impose a min-max constraint of all the nodes other than top root.
  2. In the recursion, it also pass that min-max constraint from top to bottom. In each recursion it checks left and right node not only valid for their parent (left < root < right), but also check if they are in the min-max range (The range is built up by all their ancestors).

Java Solution

 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
public class Solution {
   public boolean isValidBST(TreeNode root){
        return isValid(root Integer.MIN_VALUE, Integer.MAX_VALUE) ;
    private boolean isValid(TreeNode root, int min, int max){
        if(root==null) return true;
        if(root.val <= min || root.val >= max)
            return false;
        return isValid(root.left, min, root.val) && isValid(root.right, root.val, max);

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