题解 | #二叉树的镜像#

采用递归的思路：
1. 递归出口：如果root为空，则return （root为空则不需要翻转）
2. 递归root的左子树
3. 递归root的右子树
4. 交换root的左右子树

import java.util.*;

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

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param pRoot TreeNode类 
     * @return TreeNode类
     */
    public TreeNode Mirror (TreeNode pRoot) {

        if(pRoot == null)
            return pRoot;
        return exchangeSubTree(pRoot);
    }

    public static TreeNode exchangeSubTree(TreeNode root){
        if(root == null)
            return root;

        TreeNode leftSubTree = exchangeSubTree(root.left);
        TreeNode rightSubTree = exchangeSubTree(root.right);
        root.left = rightSubTree;
        root.right = leftSubTree;
        return root;
    }
}