1class Solution {2    public int kthSmallest(TreeNode root, int k) {3        Deque<TreeNode> stack = new ArrayDeque<>();4        TreeNode current = root;5        while (current != null || !stack.isEmpty()) {6            while (current != null) {7                stack.push(current);8                current = current.left;9            }10            current = stack.pop();11            if (--k == 0) return current.val;12            current = current.right;13        }14        throw new IllegalArgumentException("k 超出节点数");15    }16}