Leetcode: 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.
    public boolean isValidBST(TreeNode root) {
        // write your code here
        return isValidBSTHelper(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
    //return if the root val is within the [min,max] range
    public boolean isValidBSTHelper(TreeNode root, int min, int max) {
        if (root == null) {
            return true;
        if (min > root.val || root.val > max) {
            return false;
        return isValidBSTHelper(root.left, min, root.val - 1) &&
               isValidBSTHelper(root.right, root.val + 1, max);



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