LeetCode 98: 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.

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/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private TreeNode pre;
    private boolean isValid;
    
    public boolean isValidBST(TreeNode root) {
        // Adopt inorder traversal to verify the validity
        pre = null;
        isValid = true;
        
        if (root == null)
            return isValid;
            
        inorder(root);
        
        return isValid;
    }
    
    private void inorder(TreeNode node)
    {
        if (node == null)
            return;
            
        inorder(node.left);
        
        // Visit center node
        if (pre == null)
            pre = node;
        else
        {
            if (pre.val >= node.val)
            {
                isValid = false;
                return;
            }
            else
                pre = node;
        }
        
        inorder(node.right);
    }
}