解法一：递归

将源节点的左孩子作为新节点的右孩子，右孩子作为新节点的左孩子，递归完成。

  1. /**
  2.  * Definition for a binary tree node.
  3.  * public class TreeNode {
  4.  *     int val;
  5.  *     TreeNode left;
  6.  *     TreeNode right;
  7.  *     TreeNode(int x) { val = x; }
  8.  * }
  9.  */
  10. class Solution {
  11.     public TreeNode mirrorTree(TreeNode root) {
  12.         if (root == null) {
  13.             return null;
  14.         }
  15.         TreeNode mirrorNode = new TreeNode(root.val);
  16.         if (root.left != null) {
  17.             mirrorNode.right = mirrorTree(root.left);
  18.         }
  19.         if (root.right != null) {
  20.             mirrorNode.left = mirrorTree(root.right);
  21.         }
  22.         return mirrorNode;
  23.     }
  24. }