class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        //非递归的中序遍历
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while(cur!=null || !stack.isEmpty()){
            while(cur!=null){
                stack.push(cur);
                cur = cur.left;
            }
            cur = stack.pop();
            res.add(cur.val);
            cur = cur.right;
        }
        return res;
    }
}