753. Cracking the Safe

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

You can keep inputting the password, the password will automatically be matched against the last n digits entered.

For example, assuming the password is "345", I can open it when I type "012345", but I enter a total of 6 digits.

Please return any string of minimum length that is guaranteed to open the box after the entire string is inputted.

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:

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

Solution

(1) Java

class Solution {
    public String crackSafe(int n, int k) {
        if (n == 0 || k == 0) {
            return "";
        }
        if (n == 1 && k == 1) {
            return "0";
        }
        char[] chars = new char[n];
        Arrays.fill(chars, '0');
        String  passwd = new String(chars);
        Set<String> cache = new HashSet<>();
        cache.add(passwd);
        return dfs(passwd, cache, n, k, (int)Math.pow(k, n));
    }

    private String dfs(String passwd, Set<String> cache, int n, int k, int total) {
        if (cache.size() == total) {
            return passwd;
        }
        for (int i = 0; i < k; i++) {
            String npasswd = passwd.substring(passwd.length()-n+1)+i;
            if (!cache.contains(npasswd)) {
                cache.add(npasswd);
                String rst = dfs(passwd+i, cache, n, k, total);
                if (rst.length() > 0) {
                    return rst;
                } else {
                    cache.remove(npasswd);
                }
            }
        }
        return "";
    }
}

753. Cracking the Safe