LeetCode [753] Cracking the Safe

 753. Cracking the Safe

Hard

There is a box protected by a password. The password is a sequence of n digits where each digit can be one of the first k digits 0, 1, ..., k-1.

While entering a password, the last n digits entered will automatically be matched against the correct password.

For example, assuming the correct password is "345", if you type "012345", the box will open because the correct password matches the suffix of the entered password.

Return any password of minimum length that is guaranteed to open the box at some point of entering it.

 

Example 1:

Input: n = 1, k = 2
Output: "01"
Note: "10" will be accepted too.

Example 2:

Input: n = 2, k = 2
Output: "00110"
Note: "01100", "10011", "11001" will be accepted too.

 

Note:

1. n will be in the range [1, 4].
2. k will be in the range [1, 10].
3. k^n will be at most 4096.

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class Solution { public String crackSafe(int n, int k) { StringBuilder pwd = new StringBuilder(new String(new char[n]).replace('\0', '0')); int total = (int)Math.pow(k, n); Set<String> visited = new HashSet<>(); visited.add(pwd.toString()); helper(pwd, total, visited, n, k); return pwd.toString(); } boolean helper(StringBuilder pwd, int total, Set<String> visited, int n, int k){ if(visited.size() == total) return true; String surfix = pwd.substring(pwd.length()-n+1); for(char c = '0'; c<'0'+k; c++){ String surfix1 = surfix + c; if(!visited.contains(surfix1)){ pwd.append(c); visited.add(surfix1); if(helper(pwd, total, visited, n, k)) return true; visited.remove(surfix1); pwd.deleteCharAt(pwd.length()-1); } } return false; } }