面试题27：二叉树的镜像

  public class TreeNode {
        int val = 0;
        TreeNode left = null;
        TreeNode right = null;
        public TreeNode(int val) {
            this.val = val;
        }
    }

  public void Mirror(TreeNode root) {
        if(root==null) {
            return;
        }
        //左右子结点交换 -- 核心代码
        TreeNode tempNode = root.left;
        root.left=root.right;
        root.right=tempNode;

        Mirror(root.left);
        Mirror(root.right);
    }