Binary Tree Preorder Traversal && Binary Tree Postorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

Given a binary tree, return the postorder traversal of its nodes' values.

对于一棵二叉树的前序和后序遍历来说，应该很熟悉了。在这里采用的是递归的方法。

如果不在IDE里面编辑的话，貌似还真是不习惯。有一些零星的小错误。

---

public class Solution {
     public ArrayList<Integer> preorderTraversal(TreeNode root)
    {
        ArrayList<Integer> preorder = new ArrayList<Integer>();
        if(root == null)return preorder;
        visit(root,preorder);
        return preorder;
       
    }
   
    public void visit(TreeNode root,ArrayList<Integer> order)
    {
        if(root == null)return;
        order.add(root.val);
        if(root.left != null)
        {
            visit(root.left,order);
        }
        if(root.right != null)
        {
            visit(root.right,order);
        }
    }
}

---

public class Solution {
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        ArrayList<Integer> postorder = new ArrayList<Integer>();
        if(root == null) return postorder;
        visit(root,postorder);
        return postorder;
    }
   
    public void visit(TreeNode root,ArrayList<Integer> order)
    {
        if(root == null)return;
        TreeNode start = root;
        if(start.left != null)
        {
            visit(start.left,order);
        }
        if(start.right != null)
        {
            visit(start.right,order);
        }
        order.add(start.val);
    }
}