Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private boolean res = true;
    private Integer t = null;
    public boolean isValidBST(TreeNode root) {
        isValid(root);
        return res;
    }

    private void isValid(TreeNode root) {
        if (root == null) {
            return;
        }
        isValid(root.left);
        if (t == null || t < root.val) {
            t = root.val;
        } else {
            res = false;
            return;
        }
        isValid(root.right);
    }
}