原二叉树:
     4
   /   \
  2     7
 / \   / \
1   3 6   9

反转后的二叉树:

     4
   /   \
  7     2
 / \   / \
9   6 3   1
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null){
            return null;
        }
        invertTree(root.left);
        invertTree(root.right);
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        return root;
    }
}