剑指：二叉树的镜像

输入树：
      8
     /     6  10
   / \ /   5  7 9 11

 [8,6,10,5,7,9,11,null,null,null,null,null,null,null,null] 
输出树：
      8
     /     10  6
   / \ /   11 9 7  5

 [8,10,6,11,9,7,5,null,null,null,null,null,null,null,null]

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