class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new LinkedList<>();
        TreeNode prev = null;
        TreeNode p = root;
        Deque<TreeNode> stack  = new LinkedList<>();
        while(p!=null||!stack.isEmpty()){
            if(p!=null){
                while(p!=null){
                    stack.push(p);
                    p = p.left;
                }
            }
            p = stack.pop();
            if(p.right==null||p.right==prev){
                res.add(p.val);
                prev = p;
                p = null;
            }
            else{
                stack.push(p);
                p = p.right;
            }
        }
    return res;
    
    }
}