题目：请完成一个函数，输入一颗二叉树，该函数输出它的镜像。二叉树节点定义如下：

class BinaryTreeNode{
        double value;
        BinaryTreeNode left;
        BinaryTreeNode right;
        public BinaryTreeNode(int value){
            this.value = value;
        }
    }

思路：前序遍历二叉树，如果有子节点，则交换
解决方案：

public class Question27 {
    static class BinaryTreeNode{
        double value;
        BinaryTreeNode left;
        BinaryTreeNode right;
        public BinaryTreeNode(int value){
            this.value = value;
        }
    }

    public static void mirrorRecursively(BinaryTreeNode node){
        if (node == null) return;
        if (node.left == null && node.right == null) return;
        BinaryTreeNode tmp = node.left;
        node.left = node.right;
        node.right = tmp;
        if (node.left != null){
            mirrorRecursively(node.left);
        }
        if (node.right != null){
            mirrorRecursively(node.right);
        }
    }

    public static void main(String[] args) {
        BinaryTreeNode pHead = new BinaryTreeNode(1);
        BinaryTreeNode pAhead = new BinaryTreeNode(3);
        BinaryTreeNode pBhead = new BinaryTreeNode(5);
        BinaryTreeNode pChead = new BinaryTreeNode(7);
        pHead.left = pAhead;
        pHead.right = pBhead;
        pBhead.left = pChead;
        mirrorRecursively(pHead);
        System.out.println(pHead.left.value);
    }
}