題目描述：
請完成一個函式，輸入一個二叉樹，該函式輸出它的映象。

解題思路：
我們先前序遍歷這棵樹的每個結點，如果這個結點有子結點，就交換它的兩個子結點。當交換完所有非葉子結點的左右子結點後，就得到了樹的映象。

這裡採用了遞迴方式和非遞迴方式。

import java.util.Stack;


public class MirrorRecursivelyTest {

    static class Node{
        int key;
        Node left;
        Node right;

        Node(int key){
            this
 
.key = key;
        }
    }

    public static void mirrorRecursively(Node root){
        if(root == null){
            return;
        }
        if(root.left == null && root.right == null){
            return;
        }

        Node node = root.left;
        root.left = root.right;
        root.right = node;

        if
 
(root.left != null){
            mirrorRecursively(root.left);
        }
        if(root.right != null){
            mirrorRecursively(root.right);
        }

    }

    //非遞迴實現，要藉助棧
    public static void mirrorNoneRecurvisely(Node root){
        Stack<Node> stack = new Stack<Node>();
        stack.add(root);
        while
 
(!stack.isEmpty()){
            Node node = stack.pop();

            if(node.left != null || node.right != null){
                Node tmp = node.left;
                node.left = node.right;
                node.right = tmp;
            }

            if(node.left != null){
                stack.push(node.left);
            }

            if(node.right != null){
                stack.push(node.right);
            }
        }

    }

    //列印樹
    public static void printTree(Node node){
        if(node == null){
            return;
        }
        System.out.println(node.key);
        printTree(node.left);
        printTree(node.right);
    }

    public static void main(String[] args) {

        Node root = new Node(8);
        Node node8 = new Node(8);
        Node node7 = new Node(7);

        root.left = node8;
        root.right = node7;

        Node node9 = new Node(9);
        node8.left = node9;

        Node node2 = new Node(2);
        node8.right = node2;

        Node node4 = new Node(4);
        Node node72 = new Node(7);
        node2.left = node4;
        node2.right = node72;

        //mirrorRecursively(root);
        mirrorNoneRecurvisely(root);
        printTree(root);

    }

}

測試資料如下圖