public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {
    public void Mirror(TreeNode root) {
        //當前節點為空，直接返回
        if(root == null)
            return;
        //當前節點沒有葉子節點，直接返回
        if(root.left == null && root.right == null)
            return;
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        //遞迴交換葉子節點
        if(root.left != null)
            Mirror(root.left);
        if(root.right != null)
            Mirror(root.right);
    }
}