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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
1 public static List<List<Integer>> generateNumber(int length, List<Integer> baned) { 2 List<List<Integer>> res = new ArrayList<List<Integer>>(); 3 List<Integer> num = new ArrayList<Integer>(); 4 HashSet<Integer> ban = new HashSet<Integer>(); 5 for(int i=0; i<baned.size(); i++) { 6 ban.add(baned.get(i)); 7 } 8 9 helper(res, num, ban, -1, length, 0);10 11 return res;12 }13 14 public static void helper(List<List<Integer>> res, List<Integer> num, HashSet<Integer> ban, int prev, int length, int pos) {15 if(pos == length) {16 res.add(new ArrayList<Integer>(num));17 return;18 }19 20 for(int i=0; i<=9; i++) {21 if(pos != 0 && i == 4) continue; 22 else if(ban.contains(i)) continue;23 else if(i == prev) continue;24 25 num.add(i);26 helper(res, num, ban, i, length, pos+1);27 num.remove(num.size()-1);28 }29 }
Telephone Number
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