原题链接: https://oj.leetcode.com/problems/validate-binary-search-tree/

public boolean isValidBST(TreeNode root) {
    if (root == null) return true;

    LinkedList<TreeNode> inorderNodes = new LinkedList<TreeNode>();
    List<Integer> vals = new ArrayList<Integer>();

    while (root != null || !inorderNodes.isEmpty()) {
        if (root != null) {
            inorderNodes.push(root);
            root = root.left;
        } else {
            TreeNode top = inorderNodes.pop(); 
            vals.add(top.val);
            root = top.right;
        }
    }

    for (int i = 1, s = vals.size(); i < s; i++) {
        if (vals.get(i) <= vals.get(i -1)) return false;
    }

    return true;
}