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.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    double tmp = Integer.MIN_VALUE-1.0;
    
    Boolean flag = true;
    
    public void IFS(TreeNode root) {
        if(root!=null) {
            if(root.left!=null) IFS(root.left);
            if(root.val<=tmp) flag = false;
            else tmp = root.val;
            if(root.right!=null) IFS(root.right);
        }
        
    }
    
    public boolean isValidBST(TreeNode root) {
        IFS(root);
        return flag;
    }
}