L11_iterativePostOrderTraversal2StackJava

class Solution {

    public List<Integer> postorderTraversal(TreeNode root) {
        Stack<TreeNode> st1 = new Stack<TreeNode>(); 
        Stack<TreeNode> st2 = new Stack<TreeNode>();
        List<Integer> postOrder = new ArrayList<Integer>();
        
        if(root == null) return postOrder; 
        
        st1.push(root); 
        while(!st1.isEmpty()) {
            root = st1.pop(); 
            st2.add(root); 
            if(root.left != null) st1.push(root.left); 
            if(root.right != null) st1.push(root.right); 
        }
        while(!st2.isEmpty()) {
            postOrder.add(st2.pop().val); 
        }
        return postOrder; 
    }
}