Telephone Number

Problem

Print all valid phone numbers of length n subject to following constraints:
If a number contains a 4, it should start with 4
No two consecutive digits can be same
Three digits (e.g. 7,2,9) will be entirely disallowed, take as input

Solution

Recursive + DFS

public static List<List<Integer>> generateNumber(int length, List<Integer> baned) {
    List<List<Integer>> res = new ArrayList<List<Integer>>();
    List<Integer> num = new ArrayList<Integer>();
    HashSet<Integer> ban = new HashSet<Integer>();
    for(int i=0; i<baned.size(); i++) {
        ban.add(baned.get(i));
    }
    
    helper(res, num, ban, -1, length, 0);
    
    return res;
}

public static void helper(List<List<Integer>> res, List<Integer> num, HashSet<Integer> ban, int prev, int length, int pos) {
    if(pos == length) {
        res.add(new ArrayList<Integer>(num));
        return;
    }
    
    for(int i=0; i<=9; i++) {
        if(pos != 0 && i == 4) continue; 
        else if(ban.contains(i)) continue;
        else if(i == prev) continue;
        
        num.add(i);
        helper(res, num, ban, i, length, pos+1);
        num.remove(num.size()-1);
    }
}