[LeetCode] Validate Binary Search Tree

Problem:

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:O(n)

public class Solution {
    public boolean isValidBST(TreeNode root) {
        if(root == null)
            return true;
        
        return isBSF(root, Integer.MIN_VALUE, Integer.MAX_VALUE);   
    }
    
    public boolean isBSF(TreeNode root, int min, int max){
        if(root == null)
            return true;
            
        if(isBSF(root.left, min, root.val) && isBSF(root.right, root.val, max) && root.val >min && root.val < max){
            return true;
        }
            return false;
    }
}