题目

方法

先将根节点的左子树连接到右边，在递归处理子节点。

    public void flatten(TreeNode root) {
        if(root == null) return;
        
        TreeNode left = root.left;
        TreeNode right = root.right;
        
        if(left != null){
            root.right = left;
            root.left = null;
            
            TreeNode rightmost = left;
            while(rightmost.right != null){
                rightmost = rightmost.right;
            }
            rightmost.right = right;
        }
        flatten(root.right);
    }