题解 | #牛群间的二叉树颠倒#

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 root TreeNode类 
     * @return TreeNode类
     */
    public TreeNode upsideDownBinaryTree (TreeNode root) {
        // write code here
        if(root==null){
            return root;
        }
        
        return reverse(root,root.left,root.right);
    }
    public TreeNode reverse(TreeNode root,TreeNode l,TreeNode r){
        if(l==null){
            return root;
        }
       
        TreeNode ans=reverse(l,l.left,l.right);
         l.left=r;
        l.right=root;
        root.left=null;
        root.right=null;
       
        return ans;
    }
        
}