[leetcode] Binary Tree Upside Down

思路：递归实现，注意翻转的规律，以左子树作为根，找到左子树最右边的节点作为原有right子树和根的父节点。

class Solution {
    public TreeNode upsideDownBinaryTree(TreeNode root) {
        if (root ==null || root.left ==null && root.right ==null)
            return root;
        TreeNode left = upsideDownBinaryTree(root.left);
        TreeNode right = upsideDownBinaryTree(root.right);
        TreeNode leftRightest = left;
        while(leftRightest.right != null){
            leftRightest = leftRightest.right;
        }
        leftRightest.left = right;
        leftRightest.right = root;
        root.left = null;
        root.right = null;
        
        return left;
    }
}