1.递归

public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        accessTree(root,res);
        return res;
    }
    public void accessTree(TreeNode root, List<Integer> res){
        if(root == null){
            return;
        }

        accessTree(root.left,res);
        accessTree(root.right,res);
        res.add(root.val);
    }

2.循环迭代

public List<Integer> postorderTraversal(TreeNode root) {

        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode prevAccess = null;
        while( root!=null || !stack.isEmpty() ){
            while (root !=null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();

            if(root.right == null || root.right == prevAccess){

                res.add(root.val);
                prevAccess = root;
                root = null;

            }else{
                stack.push(root);
                root = root.right;
            }
        }
        return res;


    }