897. 递增顺序搜索树

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class Solution {
    TreeNode r = new TreeNode();
    public TreeNode increasingBST(TreeNode root) {
        TreeNode res = r;
        dfs(root);
        return res.right;
    }

    public void dfs(TreeNode root){
        if(root == null){
            return;
        }
        dfs(root.left);
        TreeNode tmp = new TreeNode(root.val);
        r.right = tmp;
        r = tmp;

        dfs(root.right);
    }
}