/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
   // 两种方法：一种是递归，一种是中序遍历
   // 对二叉树中序遍历得到的结果是升序的，
   // 需要注意边界值，如果最左边的节点的值是最小值
    public boolean isValidBST(TreeNode root) {
      long max = Long.MAX_VALUE;
      long min = Long.MIN_VALUE;
     
      return isValid(root, min, max);
    }
        // Deque<TreeNode> stack = new LinkedList<>();
        // long inorder = Long.MIN_VALUE;
        // while (!stack.isEmpty() || root != null) {
        //     while (root != null) {
        //         stack.push(root);
        //         root = root.left;
        //     }
        //     TreeNode tmp = stack.pop();
        //     if ((long)tmp.val <= inorder) {
        //         return false;
        //     }
        //     root = tmp.right;
        //     inorder = tmp.val;
        // }
        // return true;
    // }

    public boolean isValid(TreeNode node, long min, long max) {
        if (node == null) {
            return true;
        }
        if (node.val <= min || node.val >= max) {
            return false;
        }
        return isValid(node.left, min, node.val) && isValid(node.right, node.val, max);
    } 
}