输入: [1,null,2,3]  
   1
    \
     2
    /
   3 
​
输出: [3,2,1]

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if(root != null){
            list.addAll(postorderTraversal(root.left));
            list.addAll(postorderTraversal(root.right));
            list.add(root.val);
        }
        return list;
    }
}

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        LinkedList<Integer> list = new LinkedList<>();
        if(root == null){
            return list;
        }
        LinkedList<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode node = stack.pop();
            list.addFirst(node.val);
            if(node.left!=null){
                stack.push(node.left);
            }
            if(node.right!=null){
                stack.push(node.right);
            }
        }
        return list;
    }
}