Valid Binary Search Tree

Checking only that a node exceeds its left child and is below its right child misses violations deeper in a subtree. Instead run DFS with exclusive bounds. The root starts with negative and positive infinity represented by nullable or wide bounds. A node must satisfy low < value < high; its left child receives (low, value) and right child receives (value, high). This captures every ancestor constraint. The traversal visits each node once for O(n) time and uses O(h) recursion stack space, where h may be n in a skewed tree. Use a numeric type wide enough for boundary values instead of subtracting one, and clarify duplicate policy because some BST definitions place equals consistently on one side. Test a deep cross-subtree violation, min/max integers, duplicates, null root, and skewed trees.