关于二叉树后续遍历算法的一点思考

//二叉树的后序遍历算法，java语言实现

public class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode(int x) { val = x; }
  }
  
public ArrayList<Integer> postorderTraversal(TreeNode root) {
  ArrayList<Integer> res = new ArrayList<Integer>();
  helper(root, res);//摆脱桎梏（gu）此处可以把ArrayList也作为参数传过去
  return res;
}
private void helper(TreeNode root, ArrayList<Integer> res)
{
  if(root == null)
    return;//此句话保证{}，即空二叉树情况，能够顺利运行！
  helper(root.left,res);
  helper(root.right,res);
  res.add(root.val);
}

public class Solution {
   public ArrayList<Integer> postorderTraversal(TreeNode root) {
  ArrayList<Integer> res = new ArrayList<Integer>();
  helper(root, res);
  return res;
}
private void helper(TreeNode root, ArrayList<Integer> res)
{
    if(root == null)
    return;
    //这一种可能更容易理解一点，就是一直遍历左子树，遇到null
    //试着遍历同级的右子树，然后继续递归
    TreeNode leftTree = root.left;
    if (leftTree!= null) {
        helper(leftTree,res);
    }
    TreeNode rightTree = root.right;
    if (rightTree!= null) {
        helper(rightTree,res);
    }
    res.add(root.val);//记住此处添加root的成员变量val
    }
}