Bug Report for valid-binary-search-tree

[nvd916]

Bug Report for https://neetcode.io/problems/valid-binary-search-tree

Please describe the bug below and include any steps to reproduce the bug or screenshots if possible.

Got StackOverFlowError. This code runs fine on LeetCode.

The code (Kotlin):

/**
 * Definition for a binary tree node.
 * class TreeNode(var `val`: Int) {
 *     var left: TreeNode? = null
 *     var right: TreeNode? = null
 * }
 */

class Solution {
    fun isValidBST(root: TreeNode?): Boolean {
        fun isValid(node: TreeNode, upBound: Int?, lowBound: Int?): Boolean = with(node) {
            val newUpBound = upBound?.let { minOf(it, `val`) } ?: `val`
            val newLowBound = lowBound?.let { maxOf(it, `val`) } ?: `val`
            if (left == null && right == null) return true
            if (right == null) 
                return left!!.`val` < newUpBound 
                    && (lowBound == null || left!!.`val` > lowBound) 
                    && isValid(left!!, newUpBound, lowBound)
            val rightValid = right!!.`val` > newLowBound && (upBound == null || right!!.`val` < upBound)
            if (left == null) return rightValid && isValid(right!!, upBound, newLowBound)
            val leftValid = left!!.`val` < newUpBound && (lowBound == null || left!!.`val` > lowBound)
            return leftValid && rightValid && isValid(left!!, newUpBound, lowBound) && isValid(right!!, upBound, newLowBound)
        }
        return isValid(root!!, null, null)
    }
}