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) {
            return list;
        }
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        TreeNode prev = null;
        while (cur != null || !stack.isEmpty()) {
            while (cur != null) {
                stack.push(cur);
                cur = cur.left;
            }
            //注意使用peek而不是poll
            TreeNode top = stack.peek();
            //每次不但要判断右边是否为空，还要判断右子树的节点是否为上次打印过的节点
            if (top.right == null || top.right == prev) {
                stack.pop();
                list.add(top.val);
                //定义一个prev变量，用于每次保存上次打印过的节点
                prev = top;
            } else {
                cur = top.right;
            }
        }
        return list;
    }
}