心得：

這題也是典型的遞迴，題目要求把二元樹整個翻轉過來。

問題：

Invert a binary tree.

     4
   /   \
  2     7
 / \   / \
1   3 6   9

to

     4
   /   \
  7     2
 / \   / \
9   6 3   1

答案：

1. 遞迴

   /**
    * Definition for a binary tree node.
    * public class TreeNode {
    *     public int val;
    *     public TreeNode left;
    *     public TreeNode right;
    *     public TreeNode(int x) { val = x; }
    * }
    */
   public class Solution {
       public TreeNode InvertTree(TreeNode root) {
           if(root == null){
               return null;
           }
           
           var left = root.left;
           var right = root.right;
           root.left = InvertTree(right);
           root.right = InvertTree(left);
           
           return root;
       }
   }