230. Kth Smallest Element in a BST

就是tree的非递归Inorder traversal。

https://leetcode.com/submissions/detail/46458334/

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if(root == null) {
            return res;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode cur = root;
        while(!stack.isEmpty() || cur != null) {
            if(cur != null) {
                stack.push(cur);
                cur = cur.left;
            } else {
                TreeNode tmp = stack.pop();
                res.add(tmp.val);
                cur = tmp.right;
            }
        }
        return res;
    }