145. Binary Tree Postorder Traversal

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        if (root == null) return new LinkedList();
        Stack<TreeNode> s = new Stack<TreeNode>();
        LinkedList<Integer> res = new LinkedList();
        s.push(root);
        while(!s.empty()){
            TreeNode p = s.pop();
            res.addFirst(p.val);
            //左右根，push进stack是第一次逆，addFirst又逆回来，所以还是左右根
            if(p.left != null){
                s.push(p.left);
            }
            if(p.right != null){
                s.push(p.right);
            }
        }
        return res;
    }
}