delete an extra edge from BST

Given a BST, remove an extra edge to make it BST.

    7

   / \

  5   9

 /  \ /  

3    8

public void deleteEdge(TreeNode root) {
    if(root == null) return;
    root = dfs(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
}
private TreeNode dfs(TreeNode root, int left, int right) {
    if(root == null) return null;
    if(root.val <= left || root.val >= right) return null;
    root.left = dfs(root.left, left, root.val);
    root.right = dfs(root.right, root.val, right);
    return root;
}