给你二叉树的根节点 root ，返回它节点值的 前序 遍历。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();//有返回值 要有个list
        if(root == null) {//如果根节点为空
            return list;//返回空的list
        }
        System.out.print(root.val+" ");
        list.add(root.val);

        List<Integer> left = preorderTraversal(root.left);//左边放入1个list返回
        list.addAll(left);//将左边list添加到整个list

         List<Integer> right= preorderTraversal(root.right);//右边放入1个list返回
         list.addAll(right);//将右边list添加到整个list
         return list;//返回整个list

        
    }
}

给定一个二叉树的根节点 root ，返回它的 中序 遍历。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();//有返回值 要有个list
        if(root == null) {//如果根节点为空
            return list;//返回空的list
        }
       
        List<Integer> left = inorderTraversal(root.left);//左边放入1个list返回
        list.addAll(left);//将左边list添加到整个list

         list.add(root.val);

         List<Integer> right= inorderTraversal(root.right);//右边放入1个list返回
         list.addAll(right);//将右边list添加到整个list
         return list;//返回整个list
    
    }
}

给定一个二叉树，返回它的 后序 遍历。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();//有返回值 要有个list
        if(root == null) {//如果根节点为空
            return list;//返回空的list
        }
       
        List<Integer> left =postorderTraversal(root.left);//左边放入1个list返回
        list.addAll(left);//将左边list添加到整个list

         List<Integer> right= postorderTraversal(root.right);//右边放入1个list返回
         list.addAll(right);//将右边list添加到整个list

          list.add(root.val);
         return list;//返回整个list
    
    }
}