如何递归完成反转整个树。注意临时存储置换前的left node
代码：
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public TreeNode invertTree(TreeNode root) {
if(root == null) return null;
TreeNode temp = root.left;
root.left = invertTree(root.right);
root.right = invertTree(temp);
return root;
}
}
