后续遍历 java leecode

public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        
        
         ArrayList<Integer> arry=new ArrayList<Integer>();
         if(root==null) return arry;
        Stack<TreeNode> s=new Stack<TreeNode>();
        Stack<TreeNode> s2=new Stack<TreeNode>();
        s.push(root);
        while(!s.isEmpty())
        {
            
            TreeNode t=s.pop();
            s2.push(t);
            
            if(t.left!=null) s.push(t.left); 
            if(t.right!=null) s.push(t.right);
            
            
            
        }
        while(!s2.isEmpty())
        {
            arry.add(s2.pop().val);
        }
        return arry;
            
        
        
        
        
        
        
    }
}