二叉树中非递归的前序排列如何理解

/** Inorder traversal from the root*/    public void nonRecursivePreorder() {      java.util.ArrayList list = new java.util.ArrayList();      MyStack stack = new MyStack();      if (root == null) return;      stack.push(root);      while (!stack.isEmpty()) {  //这段无法理解        TreeNode node = (TreeNode)(stack.peek());        stack.pop(); // Remove the node from the stack        list.add(node);                if (node.right != null && !list.contains(node.right)) {  // 为什么这么写          stack.push(node.right); // Add the right node to the stack        }                if (node.left != null && !list.contains(node.left)) {          stack.push(node.left); // Add the left node to the stack        }      }      for (int i = 0; i < list.size(); i++)        System.out.print(((TreeNode)(list.get(i))).element + " ");    }

 private void preorder(TreeNode root) {      if (root == null) {        return;      }      System.out.print(root.element + " ");      preorder(root.left);      preorder(root.right);    }

/** Inorder traversal from the root*/    public void nonRecursivePreorder() {      java.util.ArrayList list = new java.util.ArrayList();      MyStack stack = new MyStack();      if (root == null) return;      stack.push(root);      while (!stack.isEmpty()) {          //因为先序是根左右，所以不为空则弹出根节点        TreeNode node = (TreeNode)(stack.peek());        stack.pop(); // Remove the node from the stack        list.add(node);                if (node.right != null && !list.contains(node.right)) {            stack.push(node.right); // 有右节点则将右节点压栈        }                if (node.left != null && !list.contains(node.left)) {          stack.push(node.left); // 有左节点则将左节点压栈，因为栈是后进先出，所以这个左子节点会                                 //在下一次循环当做根节点出栈        }      }//也许是我语言匮乏，不知道该怎么解释了，这段程序基本很难再清晰了      for (int i = 0; i < list.size(); i++)        System.out.print(((TreeNode)(list.get(i))).element + " ");    }