DFS that takes the current number as a string and the number of digits left to add (n).n is 0, add the current number to the result list.k to the last digit if the result is within [0, 9].k from the last digit if the result is within [0, 9] and k is not 0 (to avoid duplicates).DFS with the updated number and n-1.DFS for each digit from 1 to 9 and an initial n value of n-1.