Validate if a given tree is a valid binary search tree.
Given the root of a binary tree, determine if it is a valid binary search tree (BST). A valid BST is defined as follows: The left subtree of a node contains only nodes with keys less than the node's key, and the right subtree contains only nodes with keys greater than the node's key.
Input: root = [5,1,4,null,null,3,6]
Output: FALSE
Explanation: Node 4 is in the right subtree of Node 5, but 4 is less than 5, which violates the BST property.

Question

Validate if a given tree is a valid binary search tree.

Given the root of a binary tree, determine if it is a valid binary search tree (BST). A valid BST is defined as follows: The left subtree of a node contains only nodes with keys less than the node's key, and the right subtree contains only nodes with keys greater than the node's key.

Input: root = [5,1,4,null,null,3,6]

Output: FALSE

Explanation: Node 4 is in the right subtree of Node 5, but 4 is less than 5, which violates the BST property.

Anonymous · Accepted Answer

public boolean isValidBST(TreeNode root) { return dfs(root, new Stack()); } boolean dfs(TreeNode root, Stack stack) { if (root == null) return true; if (!dfs(root.left, stack)) { return false; } if (!stack.isEmpty() && stack.peek() >= root.val) { return false; } stack.push(root.val); return dfs(root.right, stack); }

Validate if a given tree is a valid binary search tree.

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