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.

Solution:

Key points: find a way to pass the value of the grand parent. Got the idea from here.
Can also be solved by in-order traversing the BST and then check whether collected values are in the ascending order.

	//in Java
	public boolean isValidBST(TreeNode root) {
	if(root == null) return true;
	return isValidBSTRec(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
	}
	public boolean isValidBSTRec(TreeNode root, int min, int max){
	if(root == null) return true;
	if(root.val>min && root.val<max && isValidBSTRec(root.left, min, root.val)&&isValidBSTRec(root.right, root.val, max))
	return true;
	else
	return false;
	}

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