【剑指Offer】二叉树的镜像

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

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

class Solution {
    public TreeNode mirrorTree(TreeNode root) {
        if(root == null) return null;
        TreeNode tmp = root.left;
        root.left = mirrorTree(root.right);
        root.right = mirrorTree(tmp);
        return root;
    }
}